(A)
I recall there is a big thread on how you install HAOS under Synology VMM, so it is definitely possible. Maybe you could get some help on your disk-io issue over that thread, or read along the posts and adjust your HAOS VM config/method accordingly (you know, maybe you are close and just one check box away).
Found it:
(B)
Per the HA “Getting Started” install page there are 4 different ways to get HA going. I see you have tried the docker (not supervised) install method, but wanting more (eg custom integrations via HACS or add-on),
However, to install HACS for example does not really require the Supervisor. I found a couple of links that might help:
Download and run the installer inside the container | HACS
How to install & use Home Assistant Community Store (HACS) | TUTORIAL - Kiril Peyanski’s Blog
Can’t install HACS in Homeassistant in Docker on Synology NAS : homeassistant (reddit.com)
The HAOS VM, in the background, is actually also appliance OS plus a bunch of hidden dockers. So the HA core is a docker, the supervisor is also a docker, and there are a bunch of service dockers, all packaged nicely into the appliance OS. Furthermore, those add-ons you would get using the HAOS install, those are also dockers.
So this is not for everyone. But then you seem to know your way around dockers. And if you want maximum controls over your thing, sure you could roll those dockers yourselves and tie them together.
I recall there is a user who maintain dozens of docker install scripts on various services - an impressive list covering a lot of grounds around your smart home / home services.
Unfortunately I can’t remember his name just yet. Maybe other users passing by would have better memory (or forum search skills) than I do.
(C)
Again per the HA “Getting Started” install page, there is yet another “Supervised” install method, which would get you the Supervisor, but there will be even more hoops you would have to jump through. Essentially you need to roll a virgin debian VM inside your VMM, and then install HA in the “Supervised” way inside that debian VM, and then meet those requirements.
Again, not for everyone.