The Community user bumps questions regularly - and sometimes bumps the same question multiple times.
Shouldn't there should be less (or more) priority for questions that have already been bumped by Community?
Yes, there should be.
And there is: when a question is bumped, its "Last Activity Date" is updated to the current time, which disqualifies it from being bumped by Community again during the next 30 days.
Given there's also an element of randomness in the logic by which eligible questions are chosen, you really shouldn't see questions being bumped very often unless there are simply very few questions that qualify.