Hi Ben
If you are talking about real downwind sailing (rather than reaching) the angle for a cat rigged Mosquito is with the boom at 90 degrees to the wind with the boom out as far as it goes which should be about 10cm to 20cm off the sidestay (so the boat is only aiming about 20 degrees off the wind if that). This goes for pretty much any wind strength.

If it's really howling you may have to sheet in a bit to stop the top twisting forward of the mast and pushing you over to windward - that doesn't happen often.

Doing wide downwind angles like the Taipans, Hobies and sloop rigged Mosquitoes will feel fast but you will not get to the leeward mark as quick.

Watch the shifts in the wind and keep switching to the gybe that takes you nearest to the course for the mark.

Like Hagar said, if you are reaching then you must bear away in a gust rather than luff up. Bear away in gusts and you will often have to let out some mainsheet at the same time (never cleat the main) - the mainsheet will almost immediately need to come back in so this is where your arms get a good work out. You are trying to keep the windward hull just out of the water. You will sometimes see a skipper on trapeze steering with the tiller under their armpit so they can use both hands to constantly adjust the main.

If nosediving is a problem get both hulls in the water so there's more buoyancy up front - this means sailing lower or easing the main or getting or trap or any combination of these.