Moved cases and my PC now has a host of problems I can't solve - need second opinion

I'm sorry if this is the wrong subreddit to post in, if so please point me in the right direction. I'm at my wit's end.

Build first: https://pcpartpicker.com/user/ChrisAB/saved/Bz68K8

My PC is a frankenstein put together over a number of years. I swapped cases recently for more room (my liquid cooling had a fan on top because it physically fit, for example), increased airflow, and in hopes it would make the thing quieter. I also added a fan controller.

Ever since the PC has been doing some of the following (in order)

• Turning on with no power to my mouse/keyboard or output to my monitors (restart button doesn't work, either. Have to hold down the power button or turn off the PSU at the back)

That was originally it then after a rebuild:

• Powering up for about 5 seconds, then restarting in a constant loop
• Freezing at login screen
• Restarting after being left idle (which I thought I solved)
• And just now it just froze after being left idle for a while

My first thought is my motherboard is dying.

And to solve these issues I've done the following:

• Monitored my power - everything was steady and my PSU only a few months old as my last died.
• Monitored my heat - my CPU topped out at about 70C when I was gaming (give or take), and my ambient temperature reached 40 or so
• Took everything apart and rebuilt the PC piece by piece turning it on to see if the the mouse/keyboard issue was solved. It was for about an hour.
  • At first there were no POST codes but when we redid everything it started fine. Friend thought it was the sound card putting pressure on the motherboard but it was all fine when we were testing it, so I re-assembled and moved it back.
• No longer letting my computer shut down hard drives after 20 minutes (stopped the random restarts afaik)
• Messed with the BIOS till the cows came home but not a single change did anything.
• Replaced the CMOS battery. Three times (they all seemed to be under 3V so I bought a new one I couldn't test)
• Cried just a little

So I'm wondering if this all points to a bad motherboard or bad grounding on it, or something else entirely I'm not thinking of. I've had a friend from out of town look at it and he's far more experienced than I am but couldn't think of anything else aside from maybe power supply problems.

I work from home and need my PC (thankfully I have a less good backup) so I'm in a small panic. But I also don't have a massive disposable income so replacing my motherboard unless necessary gives me pause.

Thanks for any help you can give.