As of Electric Eel (24.10), k3s is no longer a thing.
Storage questions:
How much “essential” data (now and expected)?
What kind of redundancy/resilency is expected?
Why put Plex movies in a separate pool? (How much of this “less essential data”?)
Then we can discuss options and evaluate the number of drives.
At 3, you’re dumping raidz2 and forced into raidz1. Or 3-way mirror if 2 is the sought level of resiliency.
There are mini-ITX motherboards with 8 SATA ports, or even more. Where you’re cornering yourself is by throwing in transcoding, hence either a dGPU (Arc 310?) or an Intel iGPU.
So: Why mini-ITX?
Are there requirements or decisions (e.g. already bought hardware), you have not yet described?
You do.