[...long list of questions snipped...]
Whether it's worth learning to work on the bike or not depends on your aptitude and interest. I believe I have the aptitude, but I don't have the interest. What kind of person are you? I think someone who enjoys tinkering might get more out of a cheap e-bike than I would!
I believe the best time to buy an e-bike is when you can afford it. There'll always be something new on the horizon, so at some point you have to just pull the trigger and start enjoying yourself!
It's tough when you don't find a good bike shop to partner with. I live in a relatively small community that nonetheless has a lot of bike shops (350K people and nearly 30 bike shops!), so the thought that someone might fail to find a good bike shop is a shockingly unfamiliar concept for me. I guess all you can do is choose the best of the worst, if you decide to go the bike shop route.
For absolute best bang for the buck, I recommend a bike with a Shimano electric system. They get longer range out of a 418 Wh battery than most e-bikes can get out of a 500 Wh battery (sometimes significantly so). And yet, Shimano's 504 Wh batteries cost less than most competing 400 Wh batteries. This one-two punch makes Shimano's range and cost advantage significant, and it's both an up-front savings and a long-term savings. You'll likely get longer service life out of their batteries, and when you finally need to replace it you'll likely pay less. And a Shimano-equipped e-bike will probably cost hundreds of dollars less than a similarly equipped Bosch set-up. And Shimano has an absolutely huge dealer network for service. It's no surprise to me that some companies have switched over entirely from Bosch to Shimano (Felt comes to mind), others have mostly done so (Kona and Norco are examples), and even Trek has increased the Shimano STePS presence in their line-up. I can't think of anyone switching from Shimano to Bosch off the top of my head, in contrast.
Weight can be an important factor for some. You're more likely to find a car rack that you can carry the bike with, or to have an easier time loading it onto the bike rack on the front of a bus (all the public transit buses here have bike racks on them). Low weight is typically also a general sign of high overall quality, which might mean lower maintenance cost. Low weight also factors into good handling, and further assists range and overall enjoyment of the bike. A bike with any of the brand name players will usually be lightweight compared to the cheaper e-bikes. I'm a buy-it-once, buy-it-right kind of person. Get a good one and maintain it for a long time at low maintenance cost, and get better resale value when you're done with it, and it usually ends up costing the same or less as going no-name.
Your concern with things being discontinued isn't a big deal if you go brand name. From what I understand, Shimano (for example) guarantees a 5 year minimum parts availability period for their electric parts after they discontinue an item. You'll probably get more than that for many parts, such as batteries.
My suspicion is that if you get a good brand name bike with a Shimano or Bosch system, you could be maintaining and enjoying a decade or more into the future (or at least getting reasonable resale for it if you choose to upgrade). Your typical inexpensive hub motor e-bike? More likely not, on both counts. Hub motor companies seem to be more likely to go out of business, and more rapidly lose interest in old models as new models come out, than mid-drive companies. The past doesn't guarantee the future of course, but I doubt Shimano or Bosch is going out of business anytime soon, nor have they stopped making parts for old models yet that I'm aware of.
