Train help please

In the end, you may find you are torturing yourself about a relatively small amount of money.

You have only mentioned 2 travel days, so why a 4 day pass unless that is the minimum you can buy.

We don't know you travel dates, so it is hard to check any of your figures.

Yes, USA.

Though the Eurail pass is 4 days (there is none shorter than that), it seems like the cost for it is pretty good compared to booking direct, plus we would have flexibility if we need to make changes.

Eurail pass = 307 euros per person (plus other fees, that is the part I am not sure about, how to tell)
Non pass booking option = 450 euro per person (assuming I am not doing that wrong, heh).

Seems like the pass is the way to go, but as you mention it is for 4 days and we are only traveling on 2, so that surprises me and has me wondering what I am missing.

I am getting $350 pp 1st class for 2 people total for 4 day pass with the current discount. Eurail is telling me about 38 EUR equivalent for Eurostar reservations pp London to Brussels, the 6.50 EUR for German train reservations from there to Germany. 17 EUR for TGV reservations to Paris and perhaps another 6.50 EUR for German connecting trains. I'd just figure about $100 pp for reservation fees.

I don't know if Eurostar restricts the number of pass holders on any given train, but I have heard TGV does.

I wonder if this page on the Seat61 site might help you some. It’s specifically for train travel from London to Germany. It also has a map partway down that shows the train routes for Germany to Paris as well. https://www.seat61.com/Germany.htm (edited to have link land at the top of the page)

Regarding seat reservations, in my experience traveling with the Eurail pass, that is the only additional cost I have encountered.

For the trip you have planned, I’d anticipate you’d need seat reservations for the first leg leaving from London on the Eurostar, and for the last leg of the day you go from Germany to Paris. The other trains on your route may or may not need a reservation depending on the train combination you choose.

After reading the Eurail information regarding the Eurostar train, it appears there are limits to train reservations for Eurail pass holders. You’d want be sure there was availability for the day/time you want to travel before buying the Eurail pass.

This is what is posted at the following link:

“Eurostar wants to guarantee that the service remains available to last-minute travelers and that the train will still run on time. For these reasons, they have made seat reservations mandatory for Eurail Pass holders. They have also set a limit on the number of seats available for Eurail travelers to ensure that regular travelers are still able to purchase tickets and are not priced out.”
https://www.eurail.com/en/book-reservations/all-about-seat-reservations

It's going to be cheaper and faster to fly London to Hannover and then take the train to your final destination. Not much, but enough to still be in the argument over Eurostar. Right now I'm showing $88 (US) to Hannover and 18 Euro to Bielfeld (non-stop ICE, 50 minutes), Likewise you may want to consider flying to Paris from Hannover. Short hops inside the EU are not expensive.

The problem is that you're going through multiple countries, and they will all impact the pricing. And schedules. And you do want to reserve a seat, so make sure you include that cost.

My advise is take whatever option involves the least number of changes, because every change means a chance of unplanned delay.