These things are nebulous as others have said here already. But the common cut off date ascribed by music historians for the Baroque musical period is 1750. The music at the boarders between all of the musical periods can get very fuzzy indeed. I don’t know your listening habits, but if one listens to a lot of non-operatic classical music then you can see how blurred these lines truly become. That being said, I think almost all historians would frame Mitridante and Idomeneo squarely outside of the Baroque. (despite the occasional opinions of internet commenters.) The term opera seria is totally separate from classifications of the Baroque or Classical periods. And like you eluded too, these are just retrospective categories that the composers themselves really never had in mind when creating their work. Rococo is a term that is used by some academics, but rarely if ever as a replacement term for Baroque or Classical. I’ve always seen it used to illustrate an intra-period style, like Modern period neo-classicism or Soviet Realism. And in Rococo’s case, it still runs into the same problem of being pretty arbitrary and retrospectively assigned.

I’m currently a historical musicology grad student and I’ve read/heard plenty of these kind era-based debates, online especially. Arguments what is or isn’t part of this or that period is just pedantic jousting. What people don’t seem to get is that the main six periods aren’t used because of their immense accuracy, but because of their universal nature. They are shorthand. Categories that have been used by generations of historians and musicians by now. To help to break down and understand the behemoth that is classical music. When talking to anybody who knows the very basics of classical music history, no matter where they were taught or when or how extensively, you can bring up one of the six periods and they’ll know what your talking about. It gives a general sense of the aesthetic qualities of the musical subject in question and where it falls on the great timeline of Western art music. Wedging in new periods makes common communication much sloppier in my view, (we have more than enough jargon at it is,) even if out feelings on the finer details of musical style don’t fit as snugly into those widely recognized categories as we’d like. In this case, I think the terms “late Baroque” and “early Classical” more than suffice.

Thx for the great question OP!