FWIW I recently upgraded our 2014 Mini from El Capitan to Mojave. This is pretty big jump (skipping Sierra and High Sierra), so I did a clean install of Mojave on a SanDisk Ultra 128GB Micro SD card (roughly $20 on Amazon)
After playing around with Mojave, I began installing my 3rd party software from fresh downloads of the latest versions of: Adobe Photoshop & Premier Elements, Default Folder X, Drop Box, Firefox, Google earth Pro, Peak Hour 4, Photo Sync, Power Photos, Quicken 2017, SketchUp 2017, Turbo Tax 2017&18, VNC Viewer, Tinker Tool. When those all worked well, I paid to upgrade from Office 2007 to Office 365 and Carbon Copy Cloner from v4 to v5.
After the Sierra and High Sierra upgrade dramas, I had not planned to switch to Mojave till much later (Christmas or Valentines day). But this setup let me test out Mojave without making a commitment and I was surprised at how stable and fast Mojave was on my older hardware. BTW I wasn't interested in the new "hot" Mojave features -- I was just looking for an OS that was more up-to-date (e.g. security), at least as stable as El Capitan and not slower
So I then began load testing the Mojave OS and 3rd party apps, dual booting back to El Capitan for “real work”. When it all seemed to work well, I cloned my El Capitan startup partition from the internal HD to a boot partition on an external HD; reformatted my startup partition to APFS and cloned Mojave from the SD card to the internal drive.
The dual boot Mojave & El Capitan works very well for me because I always partition my internal drive into System/Startup and Data partitions.
GetRealBro
