macOS alternative to Meld
Have you seen this fork? https://yousseb.github.io/meld/ Maybe that will work???
As for alternatives there is FileMerge that comes with Xcode. To open it from the terminal, run opendiff.
I'm quite happy with DiffMerge, but I haven't tested it in Mojave.
DiffMerge comes with a command-line script that opens the GUI from the terminal, which is handy.