First time unraid, some questions about cache pools

Hi. I'm in the process of buying parts for my home server/NAS. Use case will be mainly Jellyfin or Plex server, torrent, pihole/adguard, Minecraft and other game servers and whatnot, probably a couple VMs at some point. And storing 4k remux movies + misc media (music, photos, ebooks, family/pool backups...) on the array.

For the array I'm most likely gonna keep it simple with XFS. There's probably not gonna be more than 2 movies streaming at a time, maybe 3 max, and if at some point reading from the array is the bottleneck I will rethink it. As long as I can stream without lag, I'd like to minimize power usage.

Then the pools. I have 3 M.2 slots (2x PCIe 4 and 1x PCIe 3). I was thinking of doing 2x 1To NVMe in mirror for appdata, dockers, VMs etc. Then, I'm wondering if it's worth using the 3rd slot as a single drive as cache pool for downloads and cached shares, with the mover putting everything on the array once a day/week? Or should I put all 3 drives in a single zRAID1 pool (or just use 2 in mirror) and use if for both appdata/docker/VM AND cache? I'm worried it'll degrade the SSDs faster if all 3 are constantly getting written/emptied on a daily basis. Also maybe it's better to keep things separate?

Other question, I'm not quite sure what's better to use for the pool(s), ZFS or BTRFS. There seems to be a lot of diverging opinions about it.

I also read a bit about L2ARC but if my ZFS pool is 2 NVMe drives I believe there's not much point to it from what I understand.

I'm not planning on doing video editing but want to be able to transcode video in real time when streaming to lower res devices or unsupported formats. I don't know if/how a pool of SSD drives might help with that, but let me know if I should plan for it.

Lastly, how does gen 3 & gen 4 m.2 interract if in the same pool? Is the speed driven by the slower one if I use a 3 SSD pool? Is it better to keep 2x gen4 in mirror and have the gen3 alone for cache or it does not matter?

Thank you for your help