Exporting emails from an IMAP server

I run my own IMAP and SMTP servers, for personal and family use. After several years, I've got quite a large volume of emails. I'd like to export the oldest ones, while keeping the ability to read them. After some time spent on the web, I found the following solution, that seems to fulfill my needs:

• use offlineimap to get a copy of all emails. I'll store this local copy on family's NAS (RAID 1 configuration), in Maildir format.
• use Balsa to read these emails, when I need to. I add a section similar to this one, to Balsa's configuration file:

[mailbox-Local]
Path=<pathToNASFolderWhereMaildirsAre>
Type=LibBalsaMailboxMaildir
Name=LocalImapEmails

• delete oldest emails, on the IMAP server, using my normal email client, namely Thunderbird.