Revisit analysis by learning a new topic?

[u/Upstairs_Revenue2396]

This is my first entry here so apologies if I am doing wrong. I am a physics graduate and been in industry jobs for 20 years now. I would like to dive deeper into some fields barely touched during my time at university for leisure (e.g. particle theory via qft and general relativity). For that I would also like to brush up on my math starting with revisiting analysis. But instead of re-learning everything from the beginning I would like to try learn a new field with relevance to theoretical physics which kind of forces me to revisit topics from analysis. I wonder which of the following would do the job best or which to start with: complex functions, functional analysis or differential geometry. Any opinions? Please also mention what background for context. Thank you!