Mac Mail Import Fail, Seeking Expert Advice

I'm using a mid-2013 Macbook Air running OS Big Sur 11.0.1 The hard drive in this Macbook Air died so I had the hard drive replaced and had to begin anew using Migration Assistant from a Time Machine back up. Everything on my replacement hard drive is as it was, except that I can't get my Mac Mail program up and running again.

When I first open the Mail program I immediately get a "Mail Message Import" message saying "Mail needs to import your messages. This might take a few minutes. You won't be able to use Mail until the import is finished. Click Continue to import your messages now, or click Cancel to import later."

Importing my previous messages into Mail is exactly what I want to do, so that my Mac Mail program will be exactly as it was before my hard drive died. I have a lot of emails (170K), and it's very important they are all together in the same place.

So when I click "Continue", Mail begins to import the messages, and I can see it is importing the messages correctly because it is loading in all my various email boxes I had previously set up on the Mail program in alphabetical order.

About halfway through the import process, at approximately 100K messages, the import quits and I get the following error message: "The import failed. An error occurred during the import. Make sure you have available space in your home folder and try again."

I've erased a number of things on my hard drive to make sure there's ample free space. Currently I have 150 GB free. The Mac Mail file with all my emails is already in the Library in the "Mail" folder and takes up about 40 GB. I can see it contains *all* my needed email files and mailboxes, including the ones that didn't load during the aborted import process. So essentially I'm trying to get all these previous emails and settings, etc. back onto the Mac Mail program so that it operates exactly as it did before my hard drive died.

If anyone has any any help for me at all it's incredibly appreciated, including advice on how to find an expert who I could pay to help me get my Mail program back to the way it was. I'm in the Toronto area but happy to work with anyone worldwide who is an expert on this stuff. Thanks so much!