When active fermentation is done and no more CO2 is being produced, I’d retire the airlock and either seal the vessel with foil or a solid rubber stopper. Then you don’t have to worry about evaporation.
Get your self some breathable silicone bungs. If it is still fermenting slowly or if the temperature goes up, the bung can burp (or should a bung fart?) out the extra gas as opposed to a stopper which would get pushed out unless held in place. If the temperature drops the lid on the bung gets pulled flat.
Thanks all for the suggestions. I’m going with kegs for long term. No glass to smash the big draw, plus better sealing. Cyser still waiting for an empty one, but 6 month old mead is now safely sitting in stainless steel.