Have you tried https://x.ai/ ? I’m not sure if it covers all you need as stated above, but it seems to have more features than Calendly. I’m just getting started with it.

Looks interesting. I’m currently looking for a Wave alternative as I live in the EU.

Wave is great, only that they don’t support customers outside of the US and Canada as of this year.

Yes re: Newton, I wished they could.
Strange about Spark. I find it the fastest. You're right about the icon making a better fit with Big Sur. ;-)
I haven't tried Airmail in ages. Back then it was too…

Looks like I should pass on Superhuman then. ;-)

Let me summarise the things that personally affect my current choices.

TWOBIRD

Great
+ Simple clean interface (like original Mailbox or currently…

One email app I haven't heard of. Will try it. Looks interesting. Thanks!

Right. I'd forgotten Sparrow! I've tried them virtually all. Airmail, Edison Mail, Google Inbox, Canary Mail, Astro, etc. Wished I had access to Superhuman but probably out of my budget range. Loved…

Mailbox.
The original modern email client.