As it is currently the grades are indeed far too wide, and a lot of the grades are not useful. B, C, D, and F are obviously not things you would aim for ever. The difference between A and AA is obscenely large, basically the difference between damn near mashing and semi competant play. AA to AAA is so large that the AA is basically a useless metric to quickly gauge score quality as it ranges from barely competant to basically an acc run. AAA to AAAA is smalller, however the quality difference is still deceptively pretty sizable (almost halving your SD to achieve from base AAA to base AAAA).
Mid grades would give base goals users would feel reward from (as is already prevalant with say 93 vs 92.5) if percents are based around goals users might aim for (near AAs, max scaling, FC, clean higher MA AAAs, clean higher RA quads). It would also make the songwheel grades more useful for users because they could quickly see score info of value for loads of songs at once. It’d be easy to see which scores maybe you could try to go for the next tier on.
Also coming from an acc player, your AAAA+ grades are way to high and aren’t useful.