I recently had to help a client move e-mails between two accounts and thunderbird out-timed multiple times. After finding out that not our firewall or the account is the issue (two gmail accounts) I looked and found some report on the mozilla forum about the issue. The workaround was to copy it first locally and then move a certain amount of emails manually.
Also I prefer the UI over current Thunderbird but I think this design could win me over:

But it seemingly needs to be configured manually to achieve that look reddit link?
Not really in the mood for trying that out.
I also need to mention I make heavy use of my personal domain and have several email accounts. It’s already a pain to manage them on the phone but sadly even there the option is gmail or outlook. Didnt look too much into alternatives but I am cagey about others having access to my SMTP password.
Yes, I heard that Microsoft attempted to hijack the (new) Mail app on Windows11 to MS news but I (hope) that they don’t do that with their Outlook Android/Windows version and only with their native Win11 client