I would use some thing to measure load,
clamp on meter is favourite, but the smart meter, the solar software, the energy plug in meter, can all do the same job.
Typically for motors we have a special overload
these allow use to set the run load and will normally trip out if the motor is stalled, the typical controls
found in the consumer unit, the first two come as B, C, and D, this is how much instantaneous load they will take, i.e. magnetic part of the trip, so a B16 will allow between 3 and 5 times the 16 amp at start up, and a C 5 - 10 and a D 10 - 20, however these are also selected to trip within the loop impedance required, so it is not a simple just swap it, some measurement need taking first. The Isolator should never trip, and the RCD is looking for an imbalance which would indicate leakage to earth.
Motors can also have electronic start systems, soft start or inverter drives, these often include overload protection, it may well be some thing has burnt out, normally smell is best tool, yes you can normally smell burn windings, the other tool is the insulation tester
but guess work is not the way to do it, or replacing bits until it works, some times one has to accept one has not got the tools, and best option is call an electrician.
