Not an immediate answer to your question. But what you describe is one of the reasons why I started the poll below and posted it earlier today. Apparantly most people up to now think an IMAP engine in DT wouldn’t be a solution or think it’s not necessary otherwise.
That said: I’ve worked around crashes like yours in the past by breaking up the import in several parts. For example by sorting the e-mails by year and placing them in a corresponding folder on the IMAP server. I then imported every year in sequence by hand.
I can concur. Either with or without plugin and full disk access, importing is very slow in comparison to the time it takes Mail to import the same e-mails from the server over the internet.