What I am honestly wishing for is

• native “Thunderbird Conversations” view for threads. Outlook has this and I guess ever other mail program, even K9Mail has native implementation.
• multi-level boolean Filters, example (if title "spam1" or "invitation") and ((mail is older than x days) or (mail comes from this recipient)) then delete
• better tablet/Laptop view in K9mail (to make it future-proof, for example a GrapheneOS tablet or laptop-like system
• dark mode in mail reader?
• fixing sensitive information sent through the autoconfiguration