I’ve used claws for like ten years and I have never felt any reason to switch, but OP’s criticism of thunderbird is that it’s too old-fashioned. Claws was more old-fashioned than thunderbird way back when I was trying out different clients, and has had no significant interface changes in the time since.
But yeah, claws is awesome. I can’t speak for power users, but as someone who doesn’t need a lot of features other than being somewhat idiot-proofed, it works great for me.
My work uses office365 and claws does not work with those mailboxes on its own, it took me a while to figure out the workaround. There’s a libre program called davmail that will allow you to access office365 emails from any client, it’s in the AUR and for Debian users I believe it’s in the native repositories.
I’m looking through this thread and the concept behind the picture is 100x more fascinating to me than the actual question. Is the number your maximum capability?
I’d estimate I go through the majority (90℅?) of each day at like a 4 bordering on 5, but at times I’ll move up to 2. It’s a pretty seamless transition, I don’t really have an explanation for why it only happens every so often. It’s not like it’s exclusively reserved for moments where it’s more useful.