Help Me Decide Which Component to Replace First for My Crashing PC

I'm having a lot of trouble with my PC crashing just a few minutes after I log into Windows. Despite trying a lot of troubleshooting methods, I still haven't fixed it. Here's what I've done so far:

1. Checked the RAM by using one stick at a time and even trying different pairs.
2. Disabled and re-enabled DOCP.
3. Reset the BIOS to default settings.
4. Swapped out the power supply for a newer one (the old PSU was about 6-7 years old; the replacement is about 3 years old).
5. Removed the GPU since I have an AMD 5600G that allows me to boot into Windows without it.
6. Changed the boot drive to a spare one I had.
7. Monitored the temperatures before the crashes, which all seemed normal.

After all of this, I even consulted a technician who made some changes:
1. He adjusted the BIOS settings by disabling Global Cstate.
2. He performed a clean installation of Windows 11.

Initially, the crashing ceased, but it returned, and now it's crashing every time I log into Windows. I've tried reinstalling Windows again, but it crashes during the installation process now and boots directly to the BIOS. I've also updated the motherboard's BIOS to the latest non-beta version. The only components I haven't changed are the CPU and motherboard.

Given all of this, I want to know which part I should consider replacing first. My options are:
1. Is the motherboard faulty?
2. Is there a setting on the motherboard that I need to tweak?
3. Could the power supply be to blame?
4. Should I change the SSD?

Here are my specs if it helps:
- Motherboard: Asus B550M-A Wifi
- CPU: AMD Ryzen 5 5600G
- GPU: Zotac GTX 1070 Mini
- RAM: Teamgroup Tforce Za 8GB CL18 DDR4 3600 (total 16GB)
- PSU: Seasonic M12II EVO 520W 80+ Bronze (bought around 2018)
- SSD: MSI M390 500GB

I'm really hoping someone can point me in the right direction!