"Supportive care" is not something I or most people know how to offer. That's my point. What does supportive care consist of? For example, I know from reading the news that pronating ventilated patients is considered preferable. What else?
But really, I think this "home care in case the hospitals are full" thing is a pipe dream. A uniquely American pipe dream. For one thing, how are you going to hydrate and feed your loved one while they're on your homemade ventilator? Do you know how to insert an IV? Do you have saline solution on-hand? Ventilated patients in the hospital are being sedated, to my understanding. Do you have an anesthetist on hand to manage that? Etc. The American impulse to independence and prepping is so silly sometimes.
The only practical use for these ventilators is in a hospital context, employed by real doctors and nurses. And then only if the number of available ventilators ends up being the bottleneck (as opposed to the amount of healthcare worker labor available or some other factor). Any other imagined use is just a safety blanket, IMO.