Personally, I believe that the ranks are multi-dimensional, which are affected by not only the game speed, but also the board size.
I think the idea of splitting ranks came from lichess (OGS developer is a big fan of lichess:) ):
https://en.lichess.org/@/FMBressac
However, you can see from the link above that, when the player hasn’t played many games in certain category, the player should be marked as provisional in that particular category. I think this should solve some problems.
Also, I think showing the category-related ranking in the games/open challenges would be better.
(Another key difference is that, lichess doesn’t have an overall rating. They only show names and titles. Therefore, they don’t have such problem of confusion. However, in go world, the titles are directly defined by the rankings, which is the reason why we still need overall ranking even if the rankings are splitted - and this causes the confusion.)