PC randomly restarts during games and stress tests. CPU, SSD and temps seem fine. PSU, GPU or motherboard?

I’m honestly running out of ideas and would appreciate some help.
Specs:
Ryzen 5 2600
RTX 2060 Super
MSI B450 Tomahawk
16GB RAM
240GB SSD (Windows)
1TB HDD
Ant Esports VP650 Plus PSU (bought in 2020)

The PC was completely fine for years. A few months ago I noticed some stuttering in games, but nothing major. Recently it started randomly restarting.
It first happened in FC 26. The game sometimes reaches the main menu, sometimes even starts a match, but eventually the PC just goes black and reboots. No BSOD, no error message.
The same thing now happens in Forza Horizon as well.

Here’s what I’ve already tried:
Replaced the CPU thermal paste.
SSD health is Good (CrystalDiskInfo).
CPU passes Cinebench.
GPU temperatures stay normal.
Lowered GPU power limit to 70% in MSI Afterburner → still restarts.
Ran FurMark and Cinebench together. At first it survived a few minutes, then later it also restarted during the combined stress test.
It has even restarted once while sitting on the Windows desktop after booting.
Event Viewer hasn’t shown anything useful apart from the expected Kernel-Power after the restart.

One thing I noticed is that when it restarts, the screen goes black for a few seconds and then I get the MSI logo again. The CPU fan doesn’t stop spinning during the reboot.
I’m trying to figure out what to replace first. At this point I’m wondering if it’s:
the PSU,
the motherboard,
or the GPU.
Has anyone seen similar symptoms? Which component would you suspect first before I start buying parts?
Thanks!