Are you saying that the Apple Mail import function did not meet your needs? https://support.apple.com/guide/mail/im ... lp1030/mac Thunderbird stores messages in mbox format, basically a text file containing all of the messages in a given folder, including any attachments, along with structured header information for each message. That Apple support article specifically refers to importing “Files in mbox format,” so it seems to address your situation.
Keep in mind that the incentive for application developers is on the import side, not on the export process. They would like to have more users of their product and would support that by providing import functionality. Exporting data might be helpful to users leaving their current application but would not be of value to the developers of that product. This would suggest the burden should be on Apple Mail to provide a way to deal with your 300+ folders and thousands of messages.
