Which car agency/option from FRA airport?

You might also want to look at Auto Europe for lots of options. Sixt always seems more expensive.
I can't offer any more inputo n types of cars, but I can tell you my young adult son and his wife are well traveled internationally but not well experienced driving in Europe, so when the gps sent them into the walled part of the city of Rothenburg this past October...well they'll have a great story to tell their grandchildren someday!

I agree with the advice to go through https://www.autoeurope.com. I found a very inexpensive deal through Wheego, a company I had not heard of. We had no trouble. The car wasn't brand new. As for what type of car, I'd say that's a personal preference. While I enjoy a nice car, I prefer to save my money by getting the smallest/cheapest one available that will fit me and my passengers and our bags.

Most important: every driver will need IDP for driving permission in Germany (see regulation).

You will book a car class, not a dedicated brand and model (ACRISS code, example Europcar). Sixt has more detailed explanation for ACRISS.

Untypical that Sixt shall be so much more expensive so far in advance. Are you sure that your price requests were identically? Did you also check Avis?

Ensure that your contract allows driving into France.

You plan to drive in hilly areas, so do not insure too low. Better include tires.

Just as an indication: on Trustpilot Budget has surprisingly very low customer rating. It is normal that rental car firms do not have the best ratings because especially unsatisfied customers do write revenge ratings.

Checked autoeurope and it seems cheaper than direct booking. Thanks for this excellent suggestion.

Few of my stretch are long, like 4 hours drive with a kid, so I wouldn't lean towards compact. But I hope any car in Midsize and Wagon should work from Europcar, via autoeurope.

My last several trips to Germany I rented from Sixt. I have been very pleased with the cars and service. I rented at the airport in Feldkirchen and returned in Frankfurt and I rented in Frankfurt and returned there.
When I was shopping for rentals, they offered good prices. Based upon my experiences, I prefer them over the other companies that I have accounts with. I would not hesitate to recommend them.
The advice to carry an IDP is sound, but in over 45 years of renting cars throughout Europe, I have never presented one, nor have I ever been asked if I was carrying one.

like 4 hours drive with a kid

Law: You need to book a child seat for every kid under 12 years and 150 cm.

The advice to carry an IDP is sound, but in over 45 years of renting
cars throughout Europe, I have never presented one, nor have I ever
been asked if I was carrying one.

In the spirit of YMMV, I was asked for an IDP at the car rental counter at FRA once and also at a car rental counter in a Berlin suburb.

If you have a Costco membership: you might want to rent your car from Costco travel. We did that several times when we lived in Germany. It was really easy. Paperwork was never an issue.

You should be aware, though that the process of renting a car in Germany is a lot different than renting it in America. First of all pick up time takes much longer - the car is inspected for every tiny defect. Build in extra time for this process. If you have inflict any damage to the car, you will be fined. Even a very small scuff. Be sure to clean out the interior of the car very well before you drop it off.

Also, parking can be quite difficult. You really have to study the signs very carefully. We speak German quite well, and our rental car was actually towed once because we did not understand what the sign said. We had to take a 100 € cab ride to the tow yard. And we were also cited by the city. Not fun!

Are you sure you really need a car?

If you have a Costco membership: you might want to rent your car from Costco travel. We did that several times when we lived in Germany. It was really easy. Paperwork was never an issue.

You should be aware, though that the process of renting a car in Germany is a lot different than renting it in America. First of all pick up time takes much longer - the car is inspected for every tiny defect. Build in extra time for this process. If you have inflict any damage to the car, you will be fined. Even a very small scuff. Be sure to clean out the interior of the car very well before you drop it off.

Also, parking can be quite difficult. You really have to study the signs very carefully. We speak German quite well, and our rental car was actually towed once because we did not understand what the sign said. We had to take a 100 € cab ride to the tow yard. And we were also cited by the city. Not fun!

Are you sure you really need a car?

Costco is definitely cheaper than all other options, but I see all Manual ones there.

Yes, I would need the car, for places we will be driving to.

I would not shy away from car rental in Germany. I can't begin to count how often I have rented there since 1980. It's been a lot. In that time, I have had one parking ticket. It was more or less a risk I chose to take deliberately. It took me all of about 1/2 hour and a visit to the local town offices to sort it out. I paid a small fine, I think about 20 DM. I stood in line for 10 minutes to speak to a clerk and settle the ticket.
A car gives you more travel flexibility. Even though regional transportation can take you into the countryside, using buses and trains can take hours vs. minutes with a car.
What I try to do is plan my trip to use trains for larger jumps (say Fra to Berlin with a stop in Erfurt), Berlin to Munich and then rent the car in Munich or as in my last rip, Feldkirchen. It does not take a command of the German language to learn the road signs and if you use a translation tool, even those with considerable text may be interpreted. A rule of thumb to consider. Parking in Europe is almost never free. You should never expect to "get away" with randomly parking. What has made driving there much easier in recent years are the excellent map apps available on phones and in rentals. These provide precise directions to parking. I do not consider my German speaking/reading proficiency to be a particular advantage when I drive. If anything, it makes a bigger difference when I travel using public transportation.

Anyone have any experience with Enterprise Car rental at Frankfurt airport for a one way?
It appears I cold do a one way to drop off in Aachen. I have rented with Enterprise in the states but not sure of the reputation in Germany for rentals.

On our 2023 trip we went through Costco, got an automatic, and the company was Enterprise. We picked up and dropped off at FRA airport. We rented an intermediate SUV and we’re given a hybrid Hyundai Tucson and it was fine. As you noted, Costco doesn’t always offer automatics.

After reserving, check back periodically to see if the price goes down. Like airline prices, rental car prices continually change. If it does, re reserve at the lower price then cancel the original reservation.

Be aware that with a Germany rented vehicle it will not have the French low emissions zone sticker you’ll need to drive in Strasbourg. To circumvent that issue, we stayed in Colmar and took the train to Strasbourg. Strasbourg was the only town in France with restrictions.

Avis/Budget use the same cars and counter. Likewise, Enterprise/Alamo use the same cars and counter.

On our 2023 trip we went through Costco, got an automatic, and the company was Enterprise. We picked up and dropped off at FRA airport. We rented an intermediate SUV and we’re given a hybrid Hyundai Tucson and it was fine. As you noted, Costco doesn’t always offer automatics.

After reserving check back periodically to see if the price goes down. Like airline prices, rental car prices continually change. If it does, re reserve at the lower price then cancel the original reservation.

I rechecked on Costco and only "Premium SUV" is automatic, with Enterprise and price is just little extra than Europcar. What options they give for insurance at counter? I have Chase Sapphire Preferred, if that can help for covering some part.

For peace of mind, we always get the zero deductible coverage. With the zero deductible we don’t need to worry about minor scratches. Some items like windshield and tire damage, and personal injury protection are separate coverages from whichever CDW you get. Selecting zero deductible coverage comes at a cost and can significantly increase the overall cost of the rental.

With some companies unless you get basic CDW (what credit cards cover) through the rental company you may be excluded from getting the zero deductible. Check with the rental company.

If you know which towns you will be visiting, look up parking lots before going over and bookmark them to Google maps or whichever navigation app you’ll be using. That way you can go straight to them and save some time. Believe it or not, most towns have free parking lots, but you need to know where they are since many are not marked. When we selected lodging we chose places that had on-site parking.