PC won’t boot (even from bootable USB), dead GPU – CPU failing or motherboard issues?

Having weird boot/display issues – possible CPU + GPU failure? Need advice

Specs:
- Ryzen 3 3200G (iGPU)
- RX 570 (removed)
- 16GB DDR4 (2x8GB, 3200 rated)
- ASUS B450 motherboard
- PSU: Corsair RM650e (just replaced old EVGA BR 500W)
- 120GB SSD (Windows) + 1TB HDD

Original issue:
PC started freezing randomly (full lockup, needed hard resets). Prior to that (months earlier), the RX 570 would ramp fans to max and crash.

Additional context:
At one point I noticed abnormally high CPU voltage in BIOS (~1.6V), so I reset BIOS to defaults. After that, things got worse – the system started crashing during boot rather than just randomly in Windows (although voltage was back to normal). I can still get into BIOS consistently without issues.

What I’ve tried:
- Replaced PSU → no change
- Removed GPU (using integrated GPU) → system still unstable
- Reset BIOS / loaded defaults
- Tested RAM sticks individually → same issue
- Tried different monitors and cables
- Disconnected HDD, tested SSD separately
- Booted from Windows USB → same failure (black screen after Windows logo)

Current behaviour:
- With iGPU:
- BIOS always accessible and looks normal
- Outside BIOS I sometimes get green horizontal artifacts
- Always crashes/freezes when booting Windows or USB installer (starts loading then dies and gives back lit black screen)

- With RX 570 installed:
- No signal at all (monitor detects nothing)

Conclusion so far:
- PSU, SSD, OS, cables likely ruled out
- RX 570 appears dead
- iGPU appears unstable under load (but BIOS is fine)

Questions:

1. Does this point more to CPU (iGPU) failure or motherboard issue?
2. Is it plausible both GPU and CPU degraded over time (possibly due to earlier voltage instability)?
3. Would installing a new GPU likely fix everything?
4. Would a Ryzen 5600G/5600GT be a good temporary solution to run without a dedicated GPU?

Appreciate any help — trying to avoid replacing the wrong part (again).