I stumbled on this stupid-simple build that will kick out extra U-235 with splitter priorities instead of using chests and circuit counters.
It takes a bit for it to "settle in" and may kick out some U-238 during startup. Probably not an ideal build for early game as it takes quite a bit of uranium to get belt "back pressure" correct.
It feels like forgetting a semicolon, I created the exact same design, except I output both rocks with the same inserter and had half a lane of mixed until it got to the output chest, where I proceeded to use circuits and overflow chests to only need to sort everything once.
Mine works, so whenever I restart, i know how to improve it using a technique I already use in a bunch of other things