(OFFTOPIC!)

I would not recommend that with any email clients. Most are built with the assumption that you have around 3 to 8 accounts. UI, speed, and configuration may become an issue. Esp. the unified inbox in Thunderbird was slow in my personal use.

What are these mailboxes? Are they changing a lot? That's also a factor to decide in your setup.

If you have hundreds of mailboxes, and you're posting on HN here, chances are you are technically competent. I would recommend a local IMAP server like Dovecot or Stalwart installed as Docker, and then fetchmail or similar to pull (copy) all the mail into a single inbox. And then your email client uses only that one account which has all emails.