Ummm...
1 - Many driers now do use open-coil elements This I do not like re fluff & fire hazard - I too prefer mineral insulated.
2 - One RCD may be sensititised by other appliances An RCD typically trips at 23mA - not 30mA. If enough appliances leak
1-2mA then they can get up to about 10-12mA earth leakage, only needs
12mA on the tumble dryer or anything for nuisance tripping.
3 - If you have sockets without RCD, buy a plug-in RCD & test Stick the dryer on a plug-in RCD into non-RCD sockets. If you need to use an extension lead it MUST be 1.25mm and FULLY unwound because a dryer will turn it into a molten glued-together mess (or fire).
4 - If the CU is a split-load, add RCBOs on non-RCD side For example lights down, kitchen sockets, outside socket, etc.
5 - If the CU is 1-RCD re whole-house or TT, again RCBOs Basically provide individual RCBO protection where possible. Particularly for kitchen or utility room - so they don't trip the rest.
It could be the AC fan with damp fluff, or the motor - thus far it sounds like the heater. Change the heater and see how things go - if you can strip the machine and get the fluff out of it.
The fluff is in fact lint and it is highly flammable, worth having a smoke alarm above any dryer because if the belt snaps there is a time before the overheat sensors kick in. I am not convinced the overheat thermostat will kick in before any lint around the heater has gone up in smoke.