If you are asking if not updating Windows will make your Mint system insecure, the answer is no. At least to me an exploit leveraging an unmounted Windows partition is unheard of. It will of course make your Windows system less secure for the 2% of the time you do use it. Another side effect of updating it is that it may break your dual booting.
I use PDF Arranger a lot for that
I switched recently from neomutt to aerc and yes, if you want less complicated configuration, it’s a great pick. I find it less buggy and just best designed overall.
I never tried Gmail or Exchange on it, but this should have some helpful info on that: https://man.sr.ht/~rjarry/aerc/providers/
After some manual reinstalls and much repetition, I’ve been using a custom script for the past year or so, which I’m slowly open sourcing through a rewrite.
The PC itself as in hardware? Hardly… Your data is at risk. So ignoring updates for both Mint and Windows will put you at a more vulnerable position from a security standpoint.