there are two ends of security yours and them
Seems tome the fault with unsafe online banking and the 2010 data thefts
lies within the company's end as target and now T-mobile suffered data theft attacks.
meaning you can have the latest OS and browse privately an still get hacked.
as for security, that all depends on who we interact with since they can have a virus or vulnerability.
as you stated "brokers" which could be risky as well.
I used a MacBook Pro 2012 with Mojave quite often this year and felt safe
even that older system has blocking and anti-tracking software on Safari.
if you browse safe, use a secure bank and know who your brokers are, you should be fine with BigSur.
if you really need that extra layer of security, by all means see if you can upgrade to Sonoma.
hope this helped!