Distro kind of matters.
Its a server, stability is the top priority. I’d avoid Ubuntu personally, and recommend Debian and Debian variants. For your first server (and your 4th+), I’d suggest proxmox. Its Debian with virtualization and a web interface, making it easy to manage services.
I’d also mention that you do not want a DE for your server. Its a lot of overhead for no real benefit, instead you should think of it as a headless machine you can remotely access - which is also what will make proxmox handy for you.
Intel wins in transcoding in most scenarios, so an intel-based system with an iGPU is a solid target. Full transcoding of AV1 starts with 14th gen, but that includes to AV1. For decode of AV1 for transcoding, 11th+. For anything else, 7th+.
With ram you can do more with more, but leveraging virt will make the load a lot lighter. Ive got systems with 32GB, but they are also hosting whole ass VMs and not just lxc’s.
For mdadm, it stores the metadata on the drives themselves. As long as your is is separate, yes, it should be just fine and detect automatically. After install it should just be a mdadm --assemble --scan and off to the races.
That said, I will always recommend backups. You never know what will happen.
In terms of docker, yes you can back things up and move them over. That said I recommend having your docker contents that need to be persistent as part of a mount somewhere, so that instead of needing to copy them around, you just have a compose that points to a mount.
Makes backups easier too.