I’ve also recently built my own NAS and I’ve gone through similar considerations. One of my mayor decisions was not to use btrfs because it’s not recommended for Raid Z1/Raid 5. With that, I landed on ZFS and TrueNAS Scale. Note that RAID expansion should be landing in both very soon.
Things with TrueNAS were pretty easy, very quick, and everything worked nicely. However, I noticed that it was constantly accessing the disks and preventing them from spinning down. I really wanted to keep the power consumption low (<20 W idle), so I eventually decided to just go with Vanilla Debian + ZFS. I can recommend that if you want to tinker with things yourself. Otherwise, I’d recommend TrueNAS Scale.
As for migration, you might be able to create a degraded pool initially, copy over the data, and add the parity disk last. Raid expansion would ofc also help there…