I have the same problem, and honestly I can just about force the thin side of the floss between my teeth but it’ll get stuck sometimes (as the tight part is the top of the tooth), which is just not great.
The brushes, which I was excited for and bought the smallest size of, will only fit under the problem area - and are uncomfy.
So I use them to clean electronics 😅
Others have recommended other file explorers, but I use FX and rather like it.