I have a Beelink EQ12 Pro where I can put a total of 2x2TB NVMe SSDs + 1x4TB SATA SSD (not sure if more is feasible according to Beelink’s description but it is enough for me right now).
I’d like a configuration where I can lose and replace one drive without breaking anything, hotswap not needed. What would be an adapted strategy for that in terms of drive capacity, partitioning and ZFS pool? 3x2TB even if two are M.2 NVMe and one is SATA? 2x2TB + 1x4TB? Something else?
Get a cheap SSD for a boot drive. I think your only option given your constraints and your goal is use all three in a RAIDZ1. There’s no real problem with using different kinds of drives in one pool. With this setup though you’ll lose the use of 2TB of capacity on the 4TB drive. 2x2TB + 1x4TB won’t work because you’ll have no redundancy on the 4TB.
Edit: just realized you meant the 4tb could be mirrored to the 2+2 configured as a striped array. I think that could work, just have to see what truenas supports setting up. That would still meet your criteria of being able to lose one of them. Perhaps the ease of rebuilding a mirror compared to a RAIDZ1 would somewhat cancel out the doubled risk of the 2+2 failing. You’d only have 50% capacity, which in terms of number of usable tb the same as the raidz1 I suggested above.
I think personally I’d go RAIDZ1 as it seems more straight forward to set up but both of these options work.