Gmail offline backup (one time backup)

Hello. One of my gmail accounts is full. I already deleted fotos and drive, so now it's just emails. I need to delete older emails, but I don't want to lose them. What are my self-hosted options? I need to be able to read those e-mails in an easy way. (I heard of pointing Thunderbird to the backup folder. Makes sense?) I don't need continuous sync. I just need a one time sync/download to have those emails outside gmail so I can delete them in gmail. Am I overthinking this and it's just easier to do a takeout from Gmail? If so, can I selfhost an .mbox viewer? Thank you.