"With all due modesty" is not a phrase I'm familiar with. It sounds like a cross between "With all due respect" (not appropriate in this context) and "In all modesty" (could be used here).
In BrE, your second example would be "Though I say so myself, ..." or "Even if I [do] say so myself, ...".
It is unusual to give "knowledge" as someone's strong point. Everyone has knowledge of some description. Do you mean you are very good at learning new things? Do you mean that you know a lot about a lot of things?
Student or Learner