Responding your questions:
1. After 2 year of normal use, the fans get a very good amount of dust, in general all the motherboard get hard dust.
2. In good practices every year for normal thermal paste, every 2 years if you use a high end thermal paste.
3. After disassembled all the laptop, clean every piece, motherboard, screen, nvme, memory, change the thermal paste and put every piece in their place the laptop works like a charm and - 5~10 degrees more cooler.
-- to the extreme.