z-wave may be easier than expected, as I think the devices stay linked to the hardware dongle used. (This is just from memory, mind!). But if you need to change the dongle, perhaps less fun.
imo, it will be a bit of pain to get everything inside HA, but once it’s done, you’ll be inside a platform that is pretty open, and commonly used, with lots of other people (hopefully) posting up solutions to problems before you encounter them!
And because it’s software that will run on pretty much anything, you have the reassurance that even if something crazy happened, you could just reinstall an old version.
If it were me, I’d clear an entire weekend day, power off the old kit, and work away at getting HA controlling everything.
The way to tell so often seems to be if someone has called it AI or Machine Learning.
AI? “I put this through chatgpt” (or “The media department has us by the balls”)
ML? “I crunched a huge amount of data in a huge amount of ways, and found something interesting”