As far as
ECVs go, right now crowds aren't really an issue, and while it *seems* like they take a lot more space, most rental ECVs actually are (relatively) compact when compared to their in-Park rental brethren.
Having said that, you can rent larger sized ECVs that will max out, or even exceed Disney's limits on mobility device sizes. I wouldn't recommend that for obvious reasons, but there are some folks who apparently believe that "bigger is better" I guess!
It is completely possible to take a manual chair to WDW, but unless you are used to self-propelling everywhere in your daily life, it's going to be pretty tough, and most don't make it to the end of the first day before their arms/shoulders/hands feel like they have been wrung out. The whole idea of vacation is to get back home and feel at least as good as when you left, if not better, right?
Remember that at FW, you will have the internal bus system to get around the Resort, and that connects to the wider WDW transportation system, so if you decide to park your car at your cabin/campsite, you will have *2* bus transfers just to get off property. (1 on/off the internal FW bus, and 1 transfer onto the Parks/Disney Springs buses)