I would love to use the KApps for Calendar / Contacts / Mail for a better integration into KDE. But at this point Thunderbird is just so far ahead, I’m not sure KDE will be able to close the gap to make those apps a viable alternative

Some level of unification would be good. They are simply too different. I’d be more than glad to offer design help for free. The email icon looks too much like ProtonMail’s. That is not OK.