I’m pretty much with Harry on this… The first owner of my boat fitted out the (new) bare shell from scratch on an offside mooring with no facilities apart from mains electricity. It can be done, he had the practical skills (from a non-boaty background), it took him three years, he made a reasonable job of it, and for him I think the enjoyment of the fitting out was a big part of the pleasure he got from his boat. (I think he found that he didn’t enjoy cruising as much, as another three years later he sold it to me!).
I also like fixing things and tinkering. My own MO for bits of renovation and refitting (on a much smaller scale) has been to try to prepare as much as possible at home, then install or reinstall the prepared parts on the boat. Doesn’t work for everything of course, but it can save a lot of travelling time back and forth. If I was going to attempt a bigger fit out or refit, I’d look to make as much of the interior as possible in modular form from the outset, so that it could be worked on at home.