Sounds like you’re happy with your current setup, so personally I wouldn’t worry about migrating everything, just a couple and see how you like it.
As for raid, it’s a trade off. There are a bunch of different types that offer different benefits at the cost of different capacity loss. Raid 1 will halve your storage, but raid 5 is less, 1/3? I forget the exact math. It depends on how important keeping the data is. For downloaded movies, it might not be super important, if a drive dies, replace and redownload. But if you store precious memories you might be way more interested in redundancy.
I haven’t tried unraid in a long time, but I personally didn’t like it. I just run fedora on my server. 🤷♀️
If you have the ability with the tos6 I would make an os partition and a data partition, that way you could change the os easily without disturbing the data, but again I’m not sure how possible that is without trying it.
I have a similar setup, I have two servers, one with the arr stack and the other is a NAS server.
I’m my fstab I mount he network drive to say /media, then the docker containers can just have Volume: /media:/media or whatever. If you need more specifics I can look when I get home.