This is a perennial problem, and you are right to say that many councils work well, and you see here the worst examples!
There used to be the English Standards Board, which was able to adjudicate on complaints against councillors. Theoretically, it had powers to remove councillors, but they were rarely used. The difficulty with this approach was that a good many of the complaints were rather trivial. Where there are real sanctions, it is necessary for the adjudication to be thorough and fair, and that is costly.
The Conservative government swept all that away. As you say, there are no real sanctions apart from criminal law, and the police are only likely to take an interest in the most egregious cases.
Personally, I would prefer to support giving greater powers to the monitoring officer, although I do not think that would be much more effective while principal authorities remain strapped for cash by virtue of government policy.
It may seem harsh, but one has to come back to the fact that the solution is in the hands of the citizens. The vast majority of town and parish councils are relatively small, and being a councillor is not particularly onerous. It is much less work than being a member of a principal authority. If citizens are unwilling to make the small effort involved, then they have only themselves to blame.
Although, on the basis of the bad examples, one might make a case for the abolition of town and parish councils, I think that would be wrong. Overall, local councils can do a lot of good and be far more in touch with local citizens than principal authorities, who have their own problems. It would be a matter of throwing out the baby with the bathwater.
Sorry I can't offer more in the way of solutions!