From Longman Dictionary of Contemporary Englishin all modestyin all modestyspokenPROUD used when you want to talk about something good you have done, but you do not want to seem too proud I think in all modesty that I can take some small credit for the team’s success. → modesty
Examples from the Corpus
in all modesty• In all modesty, I think I've matured quite a bit since those days. 