The weakest line in most agency contracts is the one that says "ongoing campaign management". It is unfalsifiable. Nobody can tell at the end of the month whether it was done well, badly, or at all.
Specify the artefacts
A retainer should name what gets produced: how many creative variants, how many reporting calls, what response time on a request, and who exactly is doing the work. If the answer to the last one is "the team", ask again.
Specify the number
Every retainer should have one metric it is accountable to, agreed up front. Not a dashboard of twenty. One.