You don’t have to build it, shops will service the motor (I’m sure there are some that will service conversions, but I don’t know any near me anyways), they may be more efficient (depending on the type of conversion/motor design you get), etc.

I’m with you, I’d do a conversion, but it’s something you’ll have to maintain and fiddle with where as the store bought ones will normally “just work” out of the box. No annoying rattles, no tweaking the motor position because it’s rubbing on something now, etc.

In my experience this normally means the seat isn’t adjusted properly. Get a level and set it to 0° and give it another ride. If it still feels bad, make sure you’re sitting far enough back that your arms are stretched out, but not so far that your elbows are locked and that you’re sitting on the correct part of the seat. If you walk in most bike shops I suspect they’ll fit you for free too.

If you have some technical ability and are on Android I’d setup sshd on the phone (there are various apps to do this easily) and use rsync(1) or scp(1).

If you want something less technical, I use the Conversations app on my phone and Dino on my desktop for chat and sometimes send files to myself. Not ideal, but it does work pretty well for one-off smaller files.