I have a 2012 going strong too. Battery has been charged and recharged less than my work 2016 but is in much better health. Upgraded it to 16GB RAM and put a 1TB SSD in it. It has worked flawlessly (it's the i7).

Do you know if there are any issues reported running a 2012 with OpenCore Legacy Patcher? Looks an interesting project I may make use of.

Are there any actual reasons the 2012 hardware cannot run Big Sur?