As others have mentioned, Thunderbird is the least worst option (in our opinions :). I use it with CalDAV/CardDAV synced to my Nextcloud instance.

The three good options are KMail, Thunderbird, and Evolution. Everything else is CLI or lacks features like DAV.

KMail and Evolution both bring in the entire KDE/Gnome PIM suite with daemons and other programs, making them not great unless you are using Gnome or KDE - as another HN commenter said about Java web applications, you wanted a banana and instead got the entire jungle and an angry gorilla. But they do integrate very well. Thunderbird is standalone.

I tried Evolution for a week, about a month ago, and have used KMail and Thunderbird a lot.

KMail is fully-featured, with native support for CalDAV/CardDAV and 'send later.' But it's incredibly complex and easy to misconfigure. When upgrading to a new computer recently, I tried doing an export => import to transfer data, but it apparently permanently borked the KMail installation on the new computer. Tried uninstalling/reinstalling and deleting all KDEPIM-related files in ~, and it still would not work... Even on my old computer I still kept Thunderbird installed along with KMail sometimes didn't work properly.

Thunderbird is very straightforward to use and is quite stable. I use the TbSync add-on (Thunderbird has an official add-on repository like Chrome/Firefox). You'll also need the 'Provider for CalDAV & CardDAV', which adds that functionality to TbSync but is distributed as a separate add on.

Set up cal/carddav account, then go to calendar, right click the toolbar => Customize, and drag the 'Synchronize' button onto the toolbar so you can force a synchronization if needed (in addition to the timer-based background sync).

There's also a 'send later' add-on available. Aside from that, I only have a few minor issues with Thunderbird:

To switch between HTML and plain-text emails, you need to shift-click the 'compose new email' button. Can't switch in the middle of composing; you'll need to make a new email and copy-paste over. Changing the default from HTML to plain text requires going into about:config. And you can't enable/disable text wrapping on the fly for plain text emails; it's an about:config pref.

With Gmail accounts, it incorrectly lists the inbox/sent/etc. folders in a subfolder of the account (functions properly, but ugly). You have to right click the account, go to Settings => Server Settings => Advanced and set the IMAP Server Directory to '[Gmail]'.

Finally, TB has its own Spam filtering mechanism. You can't fully disable it. Even if you go to account settings and disable junk filtering for that account, it still shows a button to mark as junk and overrides the J key for that. Annoying if you are used to vim controls and press J often. Also J has the homing nub on the keycap so I like pressing it a lot...

KMail has all of this stuff built in / fixed, but is just way too complex and brittle.

Actually, after seeing your article's screenshot of Evolution with KDE titlebars, and a person replying suggesting that Evolution is good for this purpose, I'm trying it out on KDE. Never thought that would happen! I don't use signatures so hopefully this bug doesn't affect me... I did have to separately install the gnome-keyring package to get it to remember the IMAP/SMTP password but aside from that it appears to work fine.