Too soon to be thinking of getting shut of the computer tbh, likewise with being concerned about reinstalling Windows which is a bridge we can help you cross should you need to.
We cannot say for sure that the PSU is the cause of the problems but it is the weakest and most sub standard component in the computer, there is evidence of unstable voltages + the computer runs for longer without the add on GPU.
Being in lock down doesn`t help but in addition to that there is also the fact that the MB and present PSU have a non propriety ATX power connector, to test the hardware with a decent PSU would also mean the need to purchase an adaptor like the example here
Regarding the BIOS update, that was done on another MB so would normally have no real baring on this unless something got damaged in the process which being honest does not look to be the case here.
The crux of things is that the type of behaviour that you describe is typical of first of all overheating and secondly a bad power supply, your temps all being very good really does suggest that the cause is the PSU.