I'm in the same boat. I have a '12 27" iMac that runs High Sierra now. I tried to upgrade to Mojave, but there is a problem with the original Fusion drive that I'm not sure I'll be able to fix. Mojave refused to install.
My conclusion was that if I can indeed fix the Fusion drive, I'll keep it running HS. If I gotta tear into it and install an SSD to replace the Fusion, I'll upgrade to Mojave.
Most OS upgrades are at least marginally slower on older hardware, so at some point you have to quit while ahead. My personal rule of thumb is to stop upgrading two years before you would need/want to replace the computer anyway.
For my 2011 iMac that point was Sierra. Sure I probably wouldn't have had any issue with HS, but I was planning on replacing the computer within two years (now within a year) anyway and decided it was worth it to lock in end-life stability.
For a 2012 I wouldn't have though High Sierra should be the end of the line, but Mojave might be. If you have any doubts just wait until Spring and check if any early adapters have posted problems and then make an extra backup beforehand if you go ahead and upgrade.
On a computer (as opposed to iOS) you can always reinstall an earlier OS version. So you can install the new version, see if it's good, and if it isn't good then you can just go back to the older OS.