Circling back on the new fan (just in case that'll help somebody in the future!) - seems like it helped.
I've got 1300rpm 3-pin fan (Noctua NF-P12 redux-1300). You want to make sure to cut off plastic guides (there's two on one side) as it won't fit in the pin socket otherwise. Also (perhaps obvious to everybody, wasn't obvious to me) you want to use the 3 rightmost pins for a connection, the fan doesn't work with three leftmost pins. When connected, assembled and turned on - the fan is pretty quiet. Perhaps slightly louder than stock fan working at 700rpm, certainly not 2x louder. UnRaid reports ~1300rpm.
Alright time for numbers - as a reminder, before the new fan I've had 55-59C during parity sync. Now, parity sync has been running for > 3 hours and it's been hovering at 50C for all three drives, so it seems like it helped quite a bit. I won't have a chance to observe idle temps before parity sync is done and expect them to be in 40-45C range now (at ambient 28-29C) . SSD temps also seem lower at about 41C used (vs 45C).
I'll keep observing and will reply back if something changes.