Thickness is good and another thing about varnish is flexibility. One of the reasons I use a high oil varnish. I believe Captains varnish is a high oil high quality finish typical of high end varnish. If I had your boat I’d sand, clean and carefully apply several more coats of varnish. A thick coat of flexible varnish will be resistant to cracking thereby keeping water from getting under the coating, allowing water to enter the porous wood and mould to develop. Look at a varnished boat and at wood joints and where screws are often .. very often you’ll see black wood running in the direction of the wood grain. A flexible and fairly thick coating of varnish works well and it’s not that hard to apply.