Yeah, I think enough has been fixed by now with 40.06 that you can probably start a fortress no problem without running into any problems. And unless a serious bug ends up being found you should be able to transfer the save forwards, meaning you get even more bug and optimization improvements as you go along.
Its stable yes. So many bugs are gone, so many others minor. But lack of invaders is a deal breaker to many many players. DF without invasions bores me to tears.
I reverted back to 34.11 a few days back to await the 40 line getting fixed up. If you do want to play 40 versions then remove all the [FLIER] tags to fix flier pathing and therefore wild animal respawns and prepare to yawn. Seriously, after the first year goblin squad you will see very little beyond wildlife and traders (though ive spotted a few were's/fb's). Removing FLIER lets new wild animals enter the map but is not the direct cause of no invader sentients. Animals can still get stuck up trees and tie up the wildlife respawns, but can usually be scared down with the new discipline system just by putting a dwarf close-by, or build a tower to shoot them out of the tree-tops.
34.11 wasnt that bad to be honest. You've got some milestone bug fixes in that release.
Your hospital already 'works' since 0.31 though your chief medical ought have no labours other than diagnose (and other medicals) to get it running promptly, picking him based on personality traits is key, crutches should work though hospital stockpiling/candy sutures may be broke without dfhack fixes.
Your kill orders clear themselves, dwarves wont wall themselves in when constructing enclosed spaces and the military uses its carried food/drink more often while not counting as naked despite armor. Archery training does work in 34.11. A marksman squad needs a regular barracks somewhere set for training in addition to archery targets also set to train. Then they will shoot sometimes, drill melee (hammer) and other skills the rest of the time. They will not shoot without both rooms assigned (i think it still works like that tbh, i always have both rooms as a throwback and can never not get them to shoot. Absolute no-skill archers may not shoot, seems to need a little skill so i let non-hunters hunt for a season before squadding them).
You've also got the night creatures revamp (including vamps), the newest animal training system with civ knowledge and minecarts plus the hauling system revamp and claimed item expiry, all epic updates.
One of my favorite 34 updates was the fix to make immigrants after first 2 waves more often be historical figures. So migrants with some skill in a field and warriors from civs-at-war rather than dozens of randomly generated adaquate milkers/threshers.
Really all you lose is the 'active world'. Which does us no favors right now with invaders being borked somewhere as all this awesome is happening out there and the closest you can come is listening to the liaison tell you about it. In addition 34.11 is exceptionally well supported by modders, Masterwork is still running off that, so maybe an opportunity to experiment a bit while you wait for a more challenging vanilla release.