Once I put myself onto the bike I don't care all that much whether it's 15lb or 50lb.
This is a really dumb thing to say; it makes absolutely no sense. ALso you typically don't "put yourself onto the bike" but rather pedal it.
I'm not talking about pedaling, I'm talking about sitting. Because now my weight combines with the bike. The difference between 215 and 250 pounds is not that impactful.
Edit: There are some differences in weight distribution, but I clearly don't mind those because they exist while the bike is powered too. The issue at hand is the pedaling, and that depends on total weight.
As in, if not for those problems you would keep riding and not push it.
So my response is that you should just fix the gearing (or buy one with good gearing), and 50lb isn't an issue for riding.
So both issues are resolved, and you could keep riding a normal design just fine. This means the design of the Rivian is a real detriment.