Not perfectly tileable and probably not the most elegant solution but my take on reprocessing. The decider combinators will use splitters as gates depending on whether there's an excess of U-235 and only make a decision while reprocessing is running for each block to guarantee that reprocessing is always the priority process.
The combinator on the left decides on whether to siphon excess forward to further centrifuges by checking for whether belts are holding an excess of 40 U-235 "and" reprocessing is running. If both are true, then the excess is sent forward. The combinator on the right checks belts for whether there is an excess of U-235 on the belts beyond what is needed to keep reprocessing running. If there is, it's split of down the undergrounds and free to go wherever it's needed. Either way there should always be an excess of 1 U-235 per centrifuge running in the system so unless the U-238 bottlenecks this should just run without issues.