Opposites attract BL?

30-sai made Dotei Da to Mahotsukai ni Nareru rashii  (Japanese)

Not entirely opposites but one is shy and one is outgoing, and their differences play a big role in the story. There is a side couple but there's not many other characters. Highly recommend this one is sooo cute.

HIStory3: Trapped (Taiwan)

One is a police officer one is a gang leader, can't get more opposite than that, and their opposites definitely play into the story. The cast is relatively small. 

Until We Meet Again (Thai)

There is a height difference, one is more quiet and reserved and one is more sweet, but the contrast doesn't play much in the story and this one has quite a large cast so it doesn't really match your criteria but I still highly recommend it. 

All three BLs have happy endings and the main couples relationships are nontoxic. Hope this helped! 

I absolutely agree with the recommendations BUT I have to warn you, there should have been a trigger warning given for the last drama (Until we meet again) - part of the main story revolves around suicide which is a theme that not everyone can or wants to deal with. The act itself is relatively short but the story around it / the repercussions of said suicide are shown every now and then during the whole story. All in All it's a super-sweet story, I just think it is something people should be warned about since it is a sensitive topic.

If you can handle it, definitely watch the drama. It's worth the watch, you'll love it

I appreciate the warning – actually I'd kind of nixed that one because of the coffins in the trailer, but now that I know it's part of the plot and not the end of the drama I may feel more inclined to watch it. On another note, I jumped right into 30-sai made Dotei Da to Mahotsukai ni Nareru rashii  and omg! so sweet! I'm loving it, so thanks again.

Here are two from searching the opposites attract tag:

https://mydramalist.com/61873-kare-ga-boku-ni-koishita-ryuu - this one you have to have the right taste to watch. Some love it, some hate it. I, personally, thought it was okay, not one of my favorites though. A season two is supposed to be made, but has not yet happened.

https://mydramalist.com/33088-what-did-you-eat-yesterday - this one is so cute! A serious man and more cheerful man. It is a different BL from what everyone is used too because it features a middle-aged couple. But, it might not exactly fit what you are looking for because there is romance but not an overwhelming amount. It is subtle since they are just a comfortable couple with each other.