Just yesterday I took measures to keep temp down further, powersave cpu governor, always full fan speed, 12 disks go to sleep after 60 min of inactivity and I removed dust for better ventilation. My NAS/server is in the attic and today theres 37°C outside

Disks were around 50°C which is too hot

10.3.0 is from 6 years ago. Do you mean 10.10.3?

10.3.x had issues for me with ios app, chromecast, hdr, hw acceleration and other stuff I don’t remember

10.3 was barely usable

I had a 2620 v2 and got me a 2690 v2 off ebay for 20 $, 10c/20t. Best in slot, imho and dirt cheap. That’s what I would recommend. I delidded the 2620 and made it a key fob