Rental car recommendation and GPS?

Never used Sixt, I've heard good things about them. I like Autoeurope. They have a U.S. based staff if you need them. For one trip in France we rented a GPS. It was better than nothing. We just rented a car in Spain, and I just used maps.me on my phone. It works like GPS, but was more accurate and more clear with instructions. You would need to download the appropriate maps when you have wifi. I started to use it well before my trip and since my trip, its my goto over my GPS. Also, I like to have a paper map, and I review my route and destinations on google or similar before I start driving.

You can use your smart phone as a GPS without even using mobile data - doesn't even need service in Europe. Just use an app like Google Maps "offline" and download the maps ahead of time while on WiFi. You won't get traffic info but your phone will work as a GPS with turn-by-turn directions without using any data, if you prefer. Try it at home to see how it works: download your local maps "offline," then put the phone in airplane mode and navigate somewhere by phone. Works surprisingly well.

But if your phone is unlocked, buying a local SIM card for your smart phone will make the map/GPS app work better and won't cost much.

Just to be clear, we used maps.me offline (by downloading the appropriate maps ahead of time) Maps.me does have some advantages over google maps. I had a plan set up for data in Europe should we have required it, but wifi at hotels and other places was sufficient.

As mentioned by others ; car rental agencies and airlines it is hard to trust the reviews.
Those that had a bad experience review negatively but no one that has a normal experience bothers to review.

Typically at most airport locations the bigger agencies are fine: Hertz, Avis, Europcar, Sixt,
I also like to use Auto Europe however the one time I did rent a car in Lisbon airport in Portugal (this summer) I did not find them price competitive at all (not sure if just a fluke)

If you have data on your phone or some other way to access data, I prefer using the WAZE app over paying for any type of GPS unit. WAZE worked great for me in Portugal.
The highway system in Portugal is amazing and outside of the cities is almost completely empty and an easy place to go fast (apparently the locals often prefer to stay on the back roads and not pay the tolls which keeps highway traffic which is tolled light ; outside of the cities)
Encountered some scary roads in Portugal but made it through just fine. The reason I found them scary is in Portugal I found many of the really narrow, windy roads do not use the roadside mirrors that are very common in France, Italy and elsewhere.

Really appreciate all the suggestions. Thanks