Best OS for iMac 2011 27 inch?

Hi,

A few years ago I set up my mother in law with a cheap 2011 27 inch iMac from eBay (with the stock Radeon card, which amazingly hasn't given in yet). Fitted it with 12GB of RAM and an SSD. It runs High Sierra very smoothly, but obviously starting to run into compatibility issues with certain websites etc on Chrome as the browser is out of date, and also Firefox (even the Extended Support Release version).

Thinking I should upgrade the OS - what is the best choice for this device to avoid graphical issues, given it's a non-Metal card? (Especially as this is for a non-techy relative).

I currently run Sequoia on a 2012 Mac mini but I understand that has a better supported GPU, so might not be the best choice for this 2011 iMac.