How do you archive your e-mails? What format should the archive-files have? eml? mbox?

I would like to REALLY archive some older e-mail-folders. Archive seems not to be the right word, because i work with empty inboxes, so every completed or answered e-mail is still archived in a folder (with the name of the year), but i use IMAP so all archived emails are still on the server.

Now i want to EXPORT the years 2018-2019 to my selfhosted-backup-server so that i can delete the emails on the mail-server.

a) Which format make most sense? EML? mbox? Another one? Would be nice if i can search the archived stuff without too many effort. b) Should i export directly from the UI of my mailprovider (if possible) or is exporting from thunderbird-client ok.