we do a lot of high end bespoke work, we used to use two separate sole trader joiners shops to make box sash windows, oak staircases, conservatorys and other bespoke joinery. both of them packed it in about two years ago, one is now a courier, the other a driving instructor. they both said they were just scratching out a living, as you say you have premises with overheads, and that's before you've even started making anything, a slack month can soon turn into a very bad month because the bills still need paying.
we also noticed it ourselves, a customer would ask for a quote for timber and upvc, on a conservatory or a front door etc. the upvc was always cheaper..
don't know tbh, rich and brian made some good suggestions, we can also diverse to a degree.
the general builder is always busy bar a full on recession. they will replace a couple of roof tiles, plaster a bit of a ceiling or unblock a bog.
when you specialise as you do. (that is the way the old school joinery shop/timber merchant etc has gone nowadays ime) the market is a lot narrower.
have you tried gazebose and other garden furniture, they always seem to do well.