We do pretty much everything ourselves, and over the years have rebuilt engines, repainted top to bottom (including a
rig), modified interiors, and added/installed any new purchases with a few exceptions.
Things I'd tend to leave to the pros:
Rigging work unless it's a simple stay/shroud replacement (which I'd get professionally made up)
Canvas work other than simple covers
I probably wouldn't tackle a transmission rebuild.
Maintaining and doing as much work on your own boat is, to my mind, a large part of the enjoyment of it all, with the added bonus that when things inevitably to wrong somewhere, you're familiar enough with things to deal with it yourself.