In addition to the already mentioned Sakuta from Bunny Girl and Rentaro from 100GF, there are a couple others that I like:
Bravern - This one is two guys that are both strong-willed. Not really sure you can call this strictly a romance though considering all the robots and apocalypse.
Endo and Kobayashi - This is actually two romances in one. One of them (in the game world) has an assertive male character. The other (in our world) a bit less so, but it was a great series that got majorly overlooked.
Love After World Domination - A criminally underwatched series that features the Red Ranger character falling in love at first sight, and he will strive to overcome all obstacles to make it happen.
Now that you mention it, I'm not sure whether to call Bravern a romance either. Romance is kind of the focus, but it kind of isn't. Either way it's one of the greatest things I've ever seen.
Hmm... if it has to have an assertive male character I'd probably have to say Toradora or Horimiya.
It is interesting how much romance doesn't have an assertive male lead. Like my absolute favorite romance is My Dress Up Darling but I wouldn't call the male lead in that one assertive. And a lot of romance have the male lead completely panic any time a girl is on the same planet as them, lol.
I briefly considered listing Wotakoi in my comment, but then
Wotakoi spoiler
like you said, Hirotaka was assertive only when it comes to games. After the first episode, he just went with the flow and as such, Narumi was taking the lead 90% of the time.