All I do is apply 3 or 4 heavy coats of an exterior grade acrylic paint (water based) and I get around 8 years before it needs a repaint. Paint the outside and top and bottom edges where rain water can sit in the joints between the boxes. but leave the inside untreated. The bees will apply a wax coating on the inside.
Your not on your own about the poor fitting of the joints, Flows answer is it is to allow for expansion and contraction of the timber but the gaps vary and I’ve even seen boxes that couldn’t be assembled without so chisel work being done to get them together and then having to fill in the gaps, a 3mm gap in some places and none with others smacks of poor quality control in my opinion.
Welcome to the forum, some great folks here very happy to help you along.
Your profile say your in Au, a bit vague if you need to ask about something when climate is a factor, Darwin is a far cry from Hobart in climate so maybe edit that so we can help.