Thanks again @Alexey for your support.
I tried turning it off for the whole night. Did not restart normally this morning. Tried unplugging everything, and plugging all back again, did not work.
I ended up swapping my whole RPi 4B motherboard with the one a friend of mine had laying around unused and all my setup came back from the dead immediately! It’s now back online and working normally! (I just had to re-setup a static address dedicated to this new RPi’s MAC address so port forwarding works correctly)
So I guess some part of my RPi’s hardware kinda died! oO
It had been working non-stop for 3 years, I’m not sure if it’s a long time or not for a RPi which is certainly not a piece of hardware supposed to be working 24/7, but computers usually last longer than that…
I’ll carry out some more testing this weekend, reboot it with a fresh SD card and see what’s going on, maybe update the firmware who knows… But for now, it really seems like it’s a hardware issue with my RPi.