[breakfastduck]

Have you explored https://github.com/dortania/OpenCore-Legacy-Patcher?

I have used it before and, in my experience and everyone else I know who has used it, the vast majority of time the newer versions run absolutely fine with no issues. Occasionally some newer features don't work, but I'd but confident that 2015/2017 iMacs would be able to run the latest version no problem.

[MilaM]

I used Dosdude's patches to install Mojave on a 2011 Mac mini. That worked well. Thanks for mentioning OpenCore-Legacy-Pather. I have it on my radar, just didn't have the time to look into it more thoroughly.