Instead of a permanent class of cops, people might rotate through guard duty; some percent of people might have the education required to investigate crime, but to prevent the potential for a repressive system, they might get called up randomly for brief periods of time (nobody should be a fulltime cop or judge).

Often you need people sufficiently trained in dealing this such situations though. One proposal I have seen is to have “professionals” but always have them accompanied by a layperson as a civil duty who can over-rule decisions and can act as a witness (unlike a body cam that is nearly always suspiciously out of battery or turned off when police abuse happens).