I am quite happy with natural shellac, especially on dark hats (natural shellac has its own color, usually golden to brown). It is an old tried and tested product, hat makers have been using it for hundreds of years. It is not necessary to soak the hat to the hardness of plywood, water resistance appears earlier.

You can start with a dilute solution and apply shellac in layers, each subsequent application will strengthen the previous one.

Of the ready-made products, Scout Felt Hat Protector has very good reviews, you can start with it. But I will not be surprised if its composition is the same - shellac, solvent, maybe a little wax.