Go look online for an old and/or refurbished Supermicro CS846 system/case. It can be typically found for anywhere from $200-2000 (or more). Try and find an old barebones system, something like a dual Intel E5-2620v2 with 128GB RAM can usually be found for like $500-550. It has storage for 24 hot swap SAS hard drives (you will need to check your version, but you should be able to get SAS 3 speeds, with at most swapping out the backplane, but the most common backplane will support SAS 3). Just need to go grab yourself some 14-24TB hard drives (I suggest starting with a set of 6), and install TrueNAS (or as most people suggest, put a hypervisor as the barebones OS such as Proxmox or XCP-NG and run TrueNAS as a VM with passthrough of the SAS controller(s) to the VM).
You can easily mod the chassis, and get replacement parts fairly easily as Supermicro is still making some variants of it and has made tens of thousands of them in the past with plenty of supply of replacement fans, controller boards, cable harnesses, power supplies, etc...
Two modifications I would highly suggest are ensuring you have power supplies that end in SQ (which meant quiet). And I suggest pulling the internal fans and fan wall from behind the disk drive backplane and instead zip tie 3x 120mm fans together like the Nocta NF-F12 iPPC-3000 and stuff a little closed cell foam on the bottom and top of them, three across, which will fill the space of the fan wall and let cables pass below (as the closed cell foam strip at the bottom will simply conform around the cables). You can then zip tie it in place to hold securely and will not only have fans that are capable of moving 2x the CFM, but will do it 12db quieter than the original fans.
With the 6 data disks, place them all in a single column in the front, and make a RAIDZ2 vdev/disk pool. You are now protected from a double disk failure, and can either expand the disk pool with three more sets of 6 disks, or create new disk pools with the new disks. If you still need more space after that, I recommend just getting an expansion chassis such as a 846JBOD (essentially a 846 chassis stripped of everything but the disk drives, power supplies for the drives and fans) or if you really need lots of disks a 946JBOD which will fit 60 disks loaded from the top. Hard to beat these from the consumer "NAS" space with some of the largest setups only getting you 6-9 disks and will cost as much or more than one of these.