Preventing transaction fee collusion between large miners
In theory, preventing this problem is why miners exist in the first place. The sort of collusion you're describing can more generally be described as censorship: miners jointly deciding to delay certain transaction unreasonably long (up to indefinitely).

Ideally - and I'm well aware that reality is quite different now - the mining ecosystem is reasonably easy to enter for anyone, including people who feel they are being censored.

This is the difference - and the only difference - between using Proof-of-Work as a consensus mechanism and using a traditional (centralized) distributed consensus mechanism: anyone can enter, without asking anyone for permission (even anonymously). If we were fine with the set of existing mining pools to never change, we could replace it with a permissioned system.