Update:
Em Client is probably the best email client I’ve used so far - the apparently small tweaks in its UI/UX save time, and “mental energy” on a daily basis.
The address book problem was probably on my end, Em Client just made it more obvious (before it is too late I should add).
Now, this is still not article-ready (I have a lot more questions than answers), but I’m writing it down on my Q&A platform. IT helps me think, figure things out, write articles once it’s sorted out, and it may help other users even as it is (still relatively raw).
Possible problem cause for my initial contacts mess:
https://www.bikegremlin.net/threads/em-client-deduplication-carddav-bug.660/#post-2804
Managed Nextcloud provider peculiarities and prices compared (what I’ve tested so far):
https://www.bikegremlin.net/threads/nextcloud-providers-compared.663/