As above. Make sure its as airtight as possible, and get a dessicant dehumidifier. Problem solved, though with some running costs.
Insulating will help a lot, but thats not going to be so easy.
You absolutely do not want more ventilation. If its 95% RH outside, it will be 95% inside. If its ventilated you cannot possibly get any other outcome.
Steel wont rust below 65%, and nor will mould grow on wood at that either.
I keep my workshop at 50%. Nothing degrades in any way. It does, however form part of my largest expense, electricity. But its a big workshop.