Mailbox handling is difficult in Thunderbird. The file structure is not intuitive and neither are the storage locations.
I recommend the following:
For each email account:
Allow user to specify the location of the mailbox (e.g. a local filesystem folder).
Use a single file for a mailbox (e.g. myPersonalEmailAccount.tbird").
Allow the user to specify an "archive" mailbox file. A user can drag/drop emails in the GUI from the primary email inbox to the archive mailbox file. Filters can also be used for this. Ensure ALL email contents and attachments are also downloaded and moved to the archive.
Archive folder is also a singular file OR with the ability to limit file size (e.g. max 1GB before splitting) or by year.
User is able to "chain" a number of mailbox archive files associated with a master email account. For example, user may have account "MYMAIL" and archive mail files called "MYMAIL-2022, MYMAIL-2021, MYMAIL-2020". Although these archive mail files are stored on an attached NAS network share location, searches will also include these "chained" mailbox files, if available. If these files are not available, then only MYMAIL (the primary email account) is searchable.
User should be able to specify files in the chain for that account at any time. Any offline, but associated mailbox files, should be indicated as offline. This is intended to permit usability if using Thunderbird primarily from a laptop with access to other mailboxes through VPN or while at their home location.
For each mailbox file associated with an account, there should be the ability to "chain" any number of mailbox files to that account, as long as they are actually corresponding to that account. It should robustly handle when any of those chained mailbox files are not available (e.g. when an associated mailbox file is no longer available on a network share). There should be no freezes associated with timeouts.
During account setup or during subsequent configuration, the user should be prompted to specify a location to save their inbox files. Currently it's stored in an obscure location under the user's profile that no one can remember or find. The user should be able to change the location of their inbox files at any point in time for any reason.
The user should be able to save the configuration of that account to a separate configuration file so that mailboxes can be quickly rebuilt if migrating to a new computer. The mailbox storage files should likewise be re-chained into the profile after the user specifies where the mailbox files are.