Personally, while TWICE is ahead in the public relevance and popularity department, BTS is not really far behind.
They literally had a large hit last year with Dynamite. It was super successful.
Yeah, due to Japan's changes you can't compare to any past hits but it is definitely far from being a minor one.
Not to mention their fandom is extremely large. Bigger than TWICE's fandom which has been killed by JYPE.
So those two factors means they are already TWICE's equal when it comes to Japan.
The biggest "idol" group hit in Japan last year would be NiziU Make You Happy following that was Dynamite.
So I would already say BTS Is on that level. But primarily as group.
I don't think they would have the same solo potential as someone from TWICE or TVXQ would there in Japan.
It would primarily be the fandom carrying any Japanese success not so much public interest.
Not there is anything wrong with that. I personally think its better to rely on Fandom for commercial success.
Overall, I think BTS Is doing just fine. They are definitely on the same level in my opinion.