I don't believe in The Great Filter, as I disagree with it's basic premise, i.e. the Fermi paradox. The Prime Directive is the most plausible explanation.

That would require a uniformity of purpose over billions of years, not only in our galaxy but in galaxies at cosmological distances (where we could detect K3 civilizations).

It would only require such uniformity for some millennia, that is how long civilization has existed. A hundred thousand years tops, if you're to believe traditional tales and mythology ;)

We don't know if alien civilizations interfered a million years ago.

It would require a prohibition on colonizing planets on which intelligent life might arise in the future. This would have to be enforced over the 4.5 billion years Earth has existed.