I find that the attributes of sewing machines fall into two groups: one contains speed and power, and the other contains strength, rigidity and weight.
I fell into the same trap of buying a reconditioned "semi-industrial" machine, and found that it failed badly as regards rigidity. This is probably the attribute we need most, towards the end of the sail making process when we are dealing with a large heavy bundle of cloth. If the needle can be easily deflected, it will not always be travelling downwards in exactly the right relationship with the bobbin mechanism. I had to buy an industrial machine, to get this kind of rigidity, but only so that I could cope adequately with regular, stiff polyester sailcloth - I managed to complete my Clipper Canvas sail on the less able machine, despite its lack of rigidity.
But if your small domestic machine is working well on four thicknesses of Clipper Canvas, then you have enough power. As Annie says, you can ease the needle through by hand - up to a point. If you are trying to go through too many thicknesses, lack of rigidity will come to the fore, and the needle will deflect, hitting parts of the mechanism beneath the bed.
I would say, press on with the domestic machine, so long as it's producing an acceptable result.