Why can you not rewrite the initial file into something partitioned by sensor+time? Would the one time job really be that much more additional cost vs the additional complexity of multiple sensors per file?

Do you ever go back and reaggregate older data into bigger, sorted files? That is, maybe you originally partitioned by hour, but stale data is so infrequently accessed, you could roll up into partitions per week/month/whatever. Depending on the specifics, you might save some space from less file overhead and better compression statistics.