In reply to ClimberEd:
> Unfortunately my imac is also 2012 and I think it's struggling with 'computer life'. 2.9GHz processor, 8GB Ram.
Both of which are fine, if a little off best these days. The issue (as mentioned above) is almost certainly your hard drive which would be much better off being an SSD. Doing this yourself is usually possible, or you can get someone else to do it...
To find out, look up your mac on everymac.com to establish which iMac you have. Then go to ifixit.com and look up SSD replacement and see if you are going to be up to it. Depending on model it can be pretty straightforward to fiendishly difficult.
The 21.5' iMacs are buggers to upgrade the RAM on (you have to remove the logic board to access the other slot, grr...), but the 27' versions are very easy (and take up to 2x 16Gb sticks) - there's a tiny door under the display or on the back that unscrews, add RAM, reboot.
I'm pretty sure all the 2012 iMacs will run Catalina. Certainly if your current machine runs Mojave then Catalina will be fine (and so you can use an iPad as an extra screen, plus one running of the thunderbolt port).
HTH
b
Post edited at 03:47