Using my previous main rig hardware I've built a fairly high end NAS for use as VM storage over iSCSI. The only parts I've had to buy specifically for this build are the case, ECC RAM, hotswap bays + HDDs, and the PM981 read cache + Optane write cache.
Luckily Ryzen supports ECC UDIMMs so I don't have to worry about bitrot or corrupted data being written to the array. Confirmed with "dmidecode --type memory" to have multi-bit error correction. This motherboard also happens to be ideal for this, since it can reallocate x4 CPU PCI-E lanes from the 2nd x16(x8 electrically) slot to the 2nd M.2 slot. So I have one empty x16 slot with 8 lanes that I can use for an HBA card when I want to expand and end up with all 24 CPU lanes used. With an HBA card I'll have 14 SATA, and there's just enough 5.25" bays in this case to fit two 4 x 5.25" to 4 x 3.5" hot swap bays, and there's room for two 2 x 5.25" to 3 x 3.5" for 14 HDDs total. At that point I may have to reinstall FreeNAS on a small USB SSD to free up the SATA that the current SSD boot drive is using.
The Seagate enterprise capacity/exos 7e8(their new name) drives are some of the highest performance SATA drives available. Currently running them as a stripe of mirrors. The PM981 ssd has above average write endurance and is just about the right size for when I add more RAM(too much read cache actually eats up too much RAM to store headers). The Optane 905p has capacitors for enhanced power loss protection(which is why it's a 22110 size M.2) which is good since if you lose your slog(write cache) you can kill the whole array. I definitely think I need more RAM and more HDDs to improve the read performance, though with the Optane I can fully saturate a single 10Gb link with writes. Also need a 10Gb switch so I can use link aggregation.
Currently working on wiring the house for 10Gbe. People would normally go for rackmount hardware, but there's nowhere I could put a rack without it sounding like a plane is taking off in the next room. I originally had a Caselabs Mercury S5 on order because there's really not a lot of other cases that have enough 5.25" bays to fit enough hotswap bays for me. About 1-2 months before it would have arrived, they went bankrupt and I had to file a chargeback. Mountain mods was the next best option that had a lot of 5.25" bays, but the quality is honestly very far behind Caselabs. The tolerances are so loose that the screw holes for the side panels barely line up.
I wouldn't recommend watercooling a storage server like this, but I just had enough watercooling gear to do it and I've never had a problem with my previous builds.
Amazing as a slog drive for ZFS. Requires a heatsink and came with one from EKWB, and boy does it run hot.