Swiscom Bluewin already has connection problems on SMTP now and then.
Context: Swisscom is the biggest telecom provider in Switzerland. If you are customer of them, you get a free e-mail address on their ad-ridden and cumbersome Bluewin. They do provide you (somewhat hidden in support documents) their IMAP/SMTP addresses. I'm on a paid provider now, but my dad isn't and has about a hundred friends and accounts, for professional use too.
I've used Thunderbird for years, and still do. I love it.
IMAP, 30GB account, contacts and calendar synced with our Nextcloud server. Can search for a term and receive a list of emails going back years instantly.
I can open Thunderbird, search for an email from 2016, and be replying to it faster than my wife's identical PC can even finish loading the Outlook splash screen (may contain traces of hyperbole).
I've recently set up Mutt with Fetchmail and Procmail. Getting mail over IMAP (with keeping those on server), putting it into one mailbox, archiving read, segmented by year and zstd-compressed, with macros for switching between outgoing SMTP accounts.
Takes little space, works fast and is very convenient once set up.
It's a very different taste from what you are describing, though.
No. A year or two ago they did a big redesign and added a lot of functionality. Of course, whenever you have changes to a software there will be some stuffy old dudes crying about it. So everywhere you look there are people who are upset because the interface is different from what they were used to even though it is way more modern and much more useful, and better for users - especially new users.