How to feel confident with knowledge if I have not taken relevant classes during my postgraduate degree?
The classes from undergrad and what you took from them enabled you to self-study all the relevant maths on your own. That is a much more valuable thing than the knowledge on its own and should give you the confidence.
If you absolutely need the &quot;external validation&quot; to build the confidence: As a postdoc you are still in a University setting? Then why not take some time out of your schedule and take the classes now? Or take some online courses that include tests and a certificate.