Midway stops between Aschaffenburg to London

There is a pretty direct line to Brussels, not sure if that appeals, but Antwerp and Ghent are close. I assume you are taking the Eurostar to London, both cities are about an hour at most to Brussels Midi/Zuid, so unless you have an early, early, departure for London, they work well.

Brussels via the rain would be a good place for 2 to 3 days. You could take a day trip to Bruges. Then take the Eurostar to London. Dropping off the car where you rented it is the best idea. You will have extra charges if you drop it off in a different country.

Amsterdam really isn't in the line from the lovely Bavarian town that my mother's pen-pal called home during the War, Aschaffenburg.

It may work, as might other places in the Randstad, if you take the much longer Eurostar Blue from Amsterdam (currently Eurostar Blue run trains from London TO Amsterdam but the construction at Amsterdam Centraal means they they don't take passengers FROM until Brussel Midi/Zuid, using Eurostar Red (formerly Thalys) from Amsterdam Centraal to Brussels where you change to the cross channel Eurostar Blue) tp London.

You could use the Stena North Sea ferry between Hook of Holland and Harwich, with trains from Amsterdam and Rotterdam via Schiedam to Hook of Holland and from Harwich to London Liverpool Street. A smooth service with easy connections and minimal walking. I like the boat, having done it several times with my car but never on the train. Some excellent YouTube videos of the whole experience, including a good one by The Man in Seat 61. Fares are competitive train-ship-train. On the daytime rune the cabins are optional and half price, the night time cabins are mandatory.

Lodging in Amsterdam is very expensive.

Brussels is not a super great place for tourism, except for the Grand Plaza, but Brugges and Ghent are great.
You might also consider Luxembourg, where you could probably find cheap flights to London.