I have a running server and would like to clean up a bit, and merge sent folders and trash folders etc.

Is this best done via the mailbox alias plugin like mentioned here? Or is there a better way?

I prefer to have one folder listed at the client, and not "Unfortunately it also means that some clients will download the same mails to local cache multiple times." https://wiki2.dovecot.org/Plugins/MailboxAlias

dovecot-2.2.10-8.el7.x86_64 CentOS Linux release 7.5.1804 (Core)