The picture does not seem to show 6 mm² cable able to take 32 amp or any doubling up and conductors in parallel. Electric circuits are designed not thrown together, so we know the pair of MCB's limits the current to 38 amps, we know using a type AC RCD, and to get within the current rating the supply cable must be 6 mm² or more.
Using my program
I have worked out needs to have no more than 26 meters of supply cable, other wise volt drop is exceeded, and when I measure with the loop impedance meter between line and neutral I expect to see around 0.58 Ω or a PSCC of around 400 amp, since a B32 MCB needs 1.365625 Ω and a C32 0.682813 Ω then seems limiting factor is volt drop.
You say you have used 6 mm² although the picture does not seem to show that, and it would need to be supplied from a 40 amp MCB in the house which you have not confirmed, you have said you have a compressor, but not said what the in-rush is for it.
To start any electric motor takes more current than to run the motor, so we use multiple methods to limit the current draw, these extend the start time, which with a compressor is often critical, as we want it to move into run mode before it starts compressing air, so the starter is often linked to a de-load valve of some sort, it means using an inverter to reduce the start load is not as easy as with other items like pillar drill or chop saw.
One of the problems with an existing supply is to ensure it has a thick enough cable for the whole of the run, I have seen new builds with garages using 2.5 mm² in the house and 4 mm² through the garden, it was on a B16 MCB so complied, but you could not have uprated to a 32 amp. And also it was right on the edge as far as volt drop goes.