I have my 4 old 3Tb drives using BTRFS with raid0 (which is what OMV set up).
Raid 0? This means a capacity of 12 TB for 4x3TB drives. If a drive breaks, all data is gone. Wouldn't it be more sensible to use mergefs instead of Raid 0? Then only the data of the defective hard drive is gone. I don't use RAID and back up to a second NAS on a thin client and a time-delayed to an external USB hard drive, which is only turned on when backing up. Raid from Raid 1 is only for reliability, is not a backup and contributes only to data security to a limited extent. In addition, raid means increased power consumption and higher hardware costs. In contrast to Germany, electricity costs do not yet seem to play a major role in many countries.
Based on my experience with a Raspi 4, I would not use a Raspi 5 for a NAS. In addition to the Raspi, you still have to buy the case, power supply and SATA-Hat. An m.2 Sata SSD will probably be mounted on top of the Sata-Hat. I don't know if there is a hat with "real" SATA connections. You still have to "tinker" with it. Then you quickly get to 150 euros. If you don't want it to look tinkered, you have to pay more money for the case. In my opinion, there are better alternatives. As a backup NAS, I now use a Wyse 5070 with a 4TB Sata m.2 SSD.