I can understand not wanting the zwave USB stick on the synology, it seems to cause nothing but problems for others when they try it.
However, I don’t believe the Hubitat C5 included zwave built in, and required a separate zstick from Hubitat. I would verify the specs on that prior to buying anything, and if the zstick is needed, that the seller is including it with the listing. The C7 does have zwave built in.
Getting a separate RPI and Zstick seems impossible right now as well. There are RPI alternatives - https://www.youtube.com/watch?v=FXkkytHSTcI . If you get one of those capable of running docker, you can get the zstick as well and then use zwavejs2mqtt in docker https://zwave-js.github.io/zwavejs2mqtt/#/getting-started/quick-start?id=docker . That could be a better more flexible route then the hubitat, and you could use that machine for other programs/purposes as well.
If you do get the hubitat, as far as I know there are still two ways to use it with Home Assistant- this custom component.
Or MQTT
I’m sure there are others using Hubitat still that can weigh in to confirm. Also, posting on those threads to ask if the method is still working can help confirm or answer specific questions on them.