Just to share a little, I've run the big Synology boxes at work and tinkered with DIY setups at home for several years.
1. The brand name boxes are the most expensive and also the most restrictive in terms of usage flexibility. Only buy these if you have deep pockets, want the warranty and don't require it to do anything much else. Personally I've always ignored warranties for storage because lost data is lost data - just spend that money on more capacity for backups instead.
2. If you just want minimum hassle and are not "techy", choose Windows over Linux.
2b. DSM or any of the other "NAS-oriented" OS will work fine, as long as you don't require it to do more than file serving. Mine eventually doubled up as a gaming and home theater PC, so having Windows was great.
3. Whichever manufacturer you choose, always get the "NAS-grade" drives. They cost more because of anti-vibration tech, among other little bits, which is essential for any multi-drive 24/7 rig. I've been a WD loyalist for years but in recent years Seagate has redeemed themselves from the 7200.12 disaster era.
4. Your primary consideration with motherboard is number of SATA ports and how many will be disabled if you use an NVME boot drive. Absent that, or if you need even more ports, make sure it has sufficient PCIe slots for expansion RAID/SATA cards and any other hardware you need. I had to upgrade to a full ATX board because eventually it had to fit a dual-slot GPU, 2x10gbit NIC and sound card.