Hatches do not have to be closed down while underway. Sure when the weather
is nasty but not in otherwise plain conditions.
But you are right on ventilation. If you think you will not have enough, think if you can modify the dorades so that you get max out of what you have already. Different sizes and shapes have vastly different efficiency.
Look if any of the hatches can be protected with a sprayhood and then left open or ajar - on many boats this is possible with the shower hatch
On our very minute boat
we can often leave the fore hatch (fore of the mast
, on the cabin
trunk) open as our inverted dinghy
is stowed above it. It is dry there 99% of the time. This hatch opens aft.