I think you're pretty far into XY territory here. I'd recommend hanging out in r/homelab and r/zfs, read the FAQs, and then if you still have questions, maybe start out with a post explaining your high level goals and challenges.
My main question here was just what I asked about NVMe drives. Many times in my research that you recommended, people recommended using multiple NVMe drives. But even a mirror is going to be problematic: on a typical motherboard (I'm using a AMD B550 chipset), there's only 2 slots, and they're connected very differently, with one slot being much faster (PCIe4) than the other (PCIe3) and having very different latency, since the fast one connects to the CPU and the slow one goes through the chipset.
If NVMe is your only option, I'd try to find a couple used 1.92TB enterprise class drives on ebay, and go ahead and mirror those without worrying about the different performance characteristics (the pool will perform as fast as the slowest device, that's all) - but 1.92TB isn't much for a media server.
In general, I'd say consumer class SSDs aren't worth the time it'll take you to install them. I'd happily deploy almost any enterprise class SSD with 50% beat out of it over almost any brand new consumer class drive. The difference is stark - enterprise drives offer superior performance through PLP-improved latency and better sustained writes (thanks to higher quality NAND and over-provisioning), while also delivering much better longevity.
I do have 4 regular SATA spinning disks (enterprise-class), for bulk data storage, in a RAIDZ1 array. I know it's not as safe as RAIDZ2, but I thought it'd be safe enough with only 4 disks, and I want to keep power usage down if possible.
I'm using (right now) a single 512GB NVMe drive for both booting and app storage, since it's so much faster. The main data will on the spinners, but the apps themselves on the NVMe which should improve performance a lot. It's not mirrored obviously, so that's one big reason I'm asking about the NVMe slots; sticking a 2nd NVMe drive in this system would actually slow it down, since the 2nd slot is only PCIe3 and connected through the chipset, so I'm wondering if people do something different, like using some kind of adapter card for the x16 video slot. I just haven't seen any good recommendations online in this regard. For now, I'm just doing daily syncs to the raid array, so if the NVMe drive suddenly dies somehow, it won't be that hard to recover, though obviously not nearly as easy as with a mirror. This obviously isn't some kind of mission-critical system so I'm ok with this setup for now; some downtime is OK, but data loss is not.
Thanks for the advice!
Move your NVMe to the other slot, I bet you can't tell a difference without synthetic benchmarks.