I've not seen any of the ten "Ossan's Love" titles (3 Japan series, 1 Japan movie, 4 Japan specials, 1 HK remake, 1 Thai remake). Here's a convenient list of all ten in release order (I won't count #6 on this list of 11 titles because it's just a "making of" production special):
https://it.mydramalist.com/list/389wYMM4

American Fan:
I've seen Ossan's Love 2018 a long time ago, but is it a remake of Ossan's Love 2016? Should I watch the 2016 original too?
I've actually quite surprised that you rated it as high as you did. I know that it was quite popular at the time, but it seems like exactly the kind of Japanese comedy with over-the-top acting that you dislike. It's never really gotten my interest till now, but maybe I should give it a try!
My understanding is that the 2016 single episode was successful and led to the 2018 full series, i.e. one can think of it as something like a pilot episode. While I doubt it would add much for you, it's just one hour, so sure, why not?

American Fan:
And what order should I see the next titles?
Ossan's Love: Love or Dead
Ossan's Love: In The Sky (wonder why Maki isn't in this one)
And prob last is Ossan's Love Returns 2024
Yes, this seems to be the correct order. It's interesting that the three "Returns" titles (the Japanese 2024 series + the two "Returns" Japan specials) have significantly higher average MDL ratings than the earlier titles.

American Fan:
There's even a 2021 Hong Kong remake
Again, I've not personally seen any of these, but I vaguely remember Chun Mo saying that he thought the HK remake was better than the Japanese original.

American Fan:
the Thai version of Ossan's Love Returns. I don't know if I've ever seen Earth in a comedy role like this.
Despite my initial scepticism, I plan to watch it because it's Earth/Mix, and I think it was Onyx (?) who said she really loved it.

American Fan:
omg, is it all worth it? 😵💫
Yeah, I'm grappling with this same question. I guess I should watch the original at some point for historical interest (i.e. for its pivotal role in popularising BL in Japan), but I really doubt I'll ever make it through all ten titles.