Update: RTX 3000 repaired!
Good news! I got my M17x R4 back from the repair shop, and the Quadro RTX 3000 is working again. If I remember correctly, the fault was caused by a bad capacitor on the GPU.
The charging issue is still present, though. The charger works normally if I connect it before turning on the laptop, but if I plug it in while the laptop is already running on battery, the charger LED fades out and the adapter shuts off. I tested another known good 240W adapter before, and it behaved the same way.
The repair guy believes the battery may be responsible and says the motherboard is fine. My battery health is only around 13% and it’s the original battery 14 years old, so I’m not sure whether that is related or whether there’s another issue with the charging circuit.
For now, I’m happy that the RTX 3000 is back to life! I’ll test it further to see whether it remains stable under load.
Thanks to everyone who helped me troubleshoot this and offered suggestions!