Paradise are good people, though I haven't been in there for quite a while.
Have you talked to JRA? Some of the folks from the old Hollywood shop. They do not do warranty work on bikes they did not sell, but they will do bike work, on an ebike.
The issue with warranty work, with most shops, is that they are responsible for getting the work done, and the company will simply not supply the parts, and also pay extremely low labor rates for the service. It just does not make sense for them. Most will do as you found, they will do the work at standard rates, while the customer must haggle with the company for parts.
I will add here that I have done warranty work for many major companies, in the IT world, and it is not that different. Much hassle getting parts, low labor rates paid. I only do warranty jobs I can bill at my standard rate, and charge an hourly for time spent haggling for parts. One of the ways I learned good diagnostic procedures was to be able to PROVE to the company that a particular part was, in fact, defective.
In some industries, warranty labor costs are 100% absorbed by the dealer, with no company compensation whatsoever.
If you can change a flat tire, you can install a hub motor kit. You can choose from ALL available standard bicycles, choose a kit with widely available, standardized parts, get a few spares, choose a kit with EXACTLY the performance you want. My first install took less than two hours, I am not much younger than you, and not mechanically inclined, at all. I am an IT person. Recently switched to a new donor bike, entire kit installed in less than one hour, with a crescent wrench and a screwdriver.
My neighbor lady has the same kit on a trike, at 74 she loves it and it gets her to the store without having to Uber. Note on standard trikes, do NOT repeat NOT attempt to corner at anything over 8-10 mph. Maybe less.