Looked as RS device and yes it will give a soft start to SOME single phase motors. It lists those it will work with. Resistor and auto-transformers could also be used to reduce start load where the motor is of a type where one can reduce the start time.
I once used a 3kW room heater to start a 240 - 110 volt transformer from a generator with a shorting switch once started. However starting a transformer reducing inrush is very different to motor start. I worked in some out of the way places like Algeria and the Falklands and one had to try some really odd methods. However the problem is in most cases it was trial and error and to buy expensive items which may not work does not seem the way forward.
The compressor will not help. Bench grinder may help. i.e. something where to light run takes no power and it has a large lump revolving.
Resistor start will not likely help with RCD. But not all RCD's are the same. Some are far better dealing with spikes than others. I would consider using X-Pole see here
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