As far as season finales go, I think this was their best one to date. The action was split pretty nicely between the chaotic events on the bridge, Saru's mission, and Burnham and Moll inside the portal. I thought the thoroughline of the clues left by the science team came together quite nicely, as well - there was a good amount of emphasis on how concerned the scientists had been about exactly who was going to find this thing. Burnham's ultimate decision to chuck the whole thing into the black hole caught me by surprise, but seemed pretty well-reasoned.
As far as series finale's go...I think they did pretty well with what they had. The several-decade time jump often works really well to cause some self-reflection, and it was nice to see an Admiral Burnham who was so settled down. And hey, we got our "Calypso" tie-in that managed to answer very few questions, but at least they closed that circle.
It really does feel like the end of an era. This show kickstarted the barrage of Trek we've (hopefully, mostly) enjoyed over these past few years. Of all the series that have followed it, Discovery has consistently been the one I've looked forward to watching the most, as it's been the most willing to surprise and challenge me along the way. It's been, as they say, a long road.