Calendar invitations are .ics files that any calendar client can consume, it’s a different remote protocol underneath too (caldav)

I'm using it as daily driver and wouldn't switch to webmail nor Thunderbird or Evolution.

As far as Evolution goes, I don't see myself moving away from it for now. I can't afford to have reminders not showing up, or have my calendar not syncing for half a business day because something has gone wrong. Evolution is the only program that I've tried that gets both of these right, and I've tried many, many applications both native and Electron-based.

Don't get me wrong, I'd switch in a heartbeat if I discovered a superior solution. But as-is, the few issues are found, aren't enough of a motivation to switch. A few cosmetic/UX glitches and nice-to-have missing features (highlighting background entries with their category's colour, busy/free/out-of-office setting on meetings) are tolerable when the alternatives are so, SO much worse.

But those issues are solved today (and since a long time by now).

I don't know whether Kontact could be considered substantially better than Evolution. But it works fine and has a lot of features not found elsewhere. It's also quite focused on power-users I would say. For example you have configurable one-keystroke commands for all important functionality (and being a KDE application you can configure shortcuts for actually every action if you like to). It handles big inboxes in different accounts (being capable of showing an unified inbox if desired) and independently of that multiple identities. The calendar and mail components handle Exchange idiosyncrasies. The GPG stuff just works.

For people looking for a true Outlook alternative on Linux it's at least one of the things worth trying. That's why I've mentioned it.