You can certainly make the artifact spawn a bit later; however, in that case, I don't get how they are spawned. I just thought that every time you have a new combination of weapon and book, there is a roll for an artifact to appear. So in each game, I tried to cycle weapons and especially books as fast as I could.
However the problem seems to be that for any given character, there aren't really many items of significance, period. Typically, you will want one specific weapon and at most one specific book. After that, you want winged boots and an armor, and that's it. Sadly, most items I find in any given run are junk. And I'm still not sure about the books because they seem to have a proper effect only with spells. With other weapons their effect seems to be much reduced (they trigger only rarely, but I can't say that they don't trigger at all — it's a bit weird). The artifact doesn't particularly change that state of things.
With respect to the boss: I think it should have much less health and much more "procs". As of now, the bossfight is quite simple and I'd say repetitive — you hit it 10 times, 20 times and then you start thinking "will it finally die?" It looks like the fight takes something between 150 and 200 rounds, which is a lot, and in that time, you unleash at least ~30 attacks, but it can be even as much as 60.
One thing that would make sense for a dragon is dragon's breath. It would make things a bit more interesting than just running straight away from it — you would need to dodge the breath weapon to the side. There could also be a lingering effect, e. g. the red dragon would breathe fire that would keep burning for a couple of turns, and if you step into it, you lose some HP. The dragon wouldn't be affected by these fires at all, and with other monsters — I don't know but probably they should be immune to it too. The black dragon could breathe some kind of poisonous vapors with a similar effect.
Finally: I think that the main game is pretty well paced now and there seems to be little need of adding some difficulty spikes. One such spike is, in my opinion, the bats, especially when the white spheres come right after them. I would rather tone down the little dragons a bit, because they seem to be very hard to defeat (the last time I managed to do that was with the overpowered archer in v1.5, after that I always had to run away and grab the amulet). The three problems with them are: 1) they have a lot HP, 2) they hit hard (for 8!) 3) there's so many of them and they come from all directions. I think that the third point is the most important: I always get swarmed, or there is such a serious threat of being swarmed that I need to run for the amulet. Maybe it would help if you spawned just a little bit less of them and give more time before the big black dragon comes.