I have had an excellent experience with TrueNAS on my 12-year-old TS-870 Pro. As the years passed, the lack of updates caused a lot of problems running newer containers in old Docker.
With TrueNAS Scale, it’s running faster, with recent packages, and, notably, 35% less power consumption! (PSA: 35% less power consumption with TrueNAS Scale on TS-870 Pro vs QTS!)