iCloud and Apple Mail syncing issues between Mac and iPhone

I've been dealing with serious syncing issues. My contacts on my Mac aren’t syncing to iCloud, and Apple Mail isn’t syncing to my iPhone. If I delete an email on my Mac, it stays on my phone, but if I delete mail on my phone, it will delete on my Mac. I've been to the Apple Store Genius Bar multiple times, spent hours with Apple Support on calls, and even hired a Mac expert, but the issue persists. I've tried everything. Has anyone else experienced this, and how did you resolve it?

[Re-Titled by Moderator]