Exactly. Really, "fully charged" should just be conceptualised as being at a sensible safe point, with the acknowledgement that it's possible to "overcharge" the batteries to an even higher level, chemically speaking, but that all sensible devices don't let you do this.

Or you could just use all of the space for a sodium battery and fully charge it as it won't need long term storage in that state.

You'll own nothing and be happy happy happyyyyy

Then buy it. No need to rent it then.
The main focus was on flexible energy packs not on the renting, although I'd find it convenient if done right.

Wpuld you rather purchase an 80 kWh battery, alrhough you need most of the time only 20 kWh or purchase only 20 kWh and rent/swap some batteries when needed?
I'm no talking about renting all battery capacity the whole year, just the extra capacity for the 2-4 weeks in the year when long-distance rides are in the mix.

It’d be ideal if we could settle on a few sizes - kind of like how we have AA, AAA, C, D, etc. batteries. One can be for such mopeds, one larger for cars and some smaller ones to fill various otherwise empty spaces in a car.

This is precisely where we're going to get fucked, though, because the modern pathological mindset of every tech company now is to try to build their own proprietary walled fiefdom to try to lock in suckers recurring revenue sources customers and they won't make their stuff compatible with anyone else's unless the government forces them to. Maybe if we're lucky there will be a decade or two of highly public bitching (see also: the Tesla charging connector) until someone eventually capitulates.

Yup, I've been thinking along those lines as well. I can't believe that every manufacturer is doing their own standards again...

Different battery chemistries have different charging requirements. So you'd have to have more complex charger/battery interaction requirements. Not insurmountable but another layer of standardization

It wouldn't be necessary very often unless you'd want to take advantage of swapping instead of reloading.

also it's not a trivial task to engineer for swapable EV batteries, doing so comes with a whole host of disadvantages / compromises that don't make sense for most (I guess) consumers right now. It's not very different from the phone battery issue, except on a huge scale and with much more severe consequences if things go wrong