How do I fix iMessage and MMS syncing issues between iPhone and Mac?

I have done everything in this: iMessage NOT syncing across iPhone and Ma… - Apple Community

this: Messages between Mac & iPhone not syncing… - Apple Community

this: iMessage NOT syncing across iPhone and Ma… - Apple Community

this: Why are my messages not syncing between i… - Apple Community

and all the 20 other messages about this from the past few years. Is there anything new or is Apple just not going to bother to make them sync anymore? I rely on the synchronicity to work so I can function during meetings and etc. Please help! I've turned on/off, synced, started/restarted, etc till I'm red in the face. I'm about ready to throw in the towel and go buy Samsung, much as I hate it, or Chrome.

[Re-Titled by Moderator]