Not much difference between 2016 and 2019 in my experience. With O365 being the favorite for MS now, the perpetual licensed versions are getting minimal updates. Compare your O2016 versions (16.20) to O2019 (16.21), from all I have seen, it is a minor dot release with a license requirement. 2019 is basically a reason to charge folks again, something they kind of have to do for their enterprise customers who refuse to go the 365 route but are "entitled" to upgrades.
They will make 2016 obsolete with no updates other that essential security fixes. 2019 will get a few enhancements here and there to keep it reasonably comparable to O365, but I suspect 2019 may be the last perpetual licensed version of Office. The uptake on O365 is gaining momentum and supporting a single code base makes more sense.
For many many years the Mac version has been just a mirror of the power of the Windows version.
For many years, the Mac version was years ahead of the Windows version. Then, MS put most of their energy into the Windows version and Mac versions fell behind. With 2016 and 2019, they are pretty equal, but a few features remain Windows only. Same is mostly true for Win vs Mac on O365. They simply find it too compelling to push the "But it works better with Windows" mantra,
My advice would be to stick with 2016 for a year or two, then get into the O365 stream. O365 costs about the same as buying retail copies every 3 years, so overall it makes a better deal if you also get the latest updates.
They seem to offer Black Friday O365 deals, so watch for those and jump on board when a deal comes along.