Earlier quoted context omitted.
No offense, but this largely reads to me like "I don't wanna use Linux because I wanna use Mac". If you want to move to a different system, you can't expect to get the same thing. Really, the only complaints I can agree with now are 2 and 3, the rest is just invalid. You've got kdeconnect gsconnect nextcloud keepassxc many, many configurable mail clients, especially the beatiful, integrated kmail and their whole PIM…
" beatiful, integrated kmail and their whole PIM suite" Is that the one that used a MySQL database on the desktop? The disaster known as akonadi? That would frequently lock up requiring restarts? When I used KDE back in the day one thing I'd make sure was to remove PIM/akonadi/kmail completely and use Thunderbird or whatever else instead.
It's been a few months since I gave up, but IIRC Akonadi has a few storage backends.
I was looking for a GUI mail client to replace using my various webmail based interfaces. As much as I love Plasma, KMail and its ilk were painful. Akonadi still has issues all these years later, and gave me trouble setting it up. The messages produced are less than helpful.
Once KMail was running, it was fine to use, albeit with an occasional crash, just at the wrong times so it got annoying. I stopped using it.