my biggest concern here is that battlemaster is pretty heavily front-loaded.

this adds quite a bit to the fighter when they need it least (at low levels, casters don't have much more utility than their skills provide unless there are no combat encounters expected, so fighters being powerful in combat but just a little less powerful outside of it is fine) while doing comparatively little to help them when they need it most (at high levels when spellcasters can do all kinds of crazy stuff, including lots of things that are much more impressive than knocking a medium-sized target prone, the fighter is adding comparatively little).

also, it really erodes options that would grant access to maneuvers, although that could be compensated for (perhaps giving an extra die or two to the options that would normally give you access to maneuvers).