Need help with importing emails into macOS Mail from ProtonMail

Hey there Tildes, hopefully someone could help me with this. I'm having problems importing all my emails I exported from ProtonMail into macOS Mail app/iCloud.

Recently decided to move my custom domain email addresses to iCloud so I could just use the built-in Mail app on my Mac and iPhone. I backed up all of my emails with ProtonMail's own export tool. It exported every email (or maybe email thread?) as a .eml file and .json file (metadata I believe). After exporting, I switch the domain names over to iCloud. It was pretty easy. I guess I assumed all of my emails would somehow come with me? Seems silly thinking about it now. But I have a backup of all my emails!

All seemed fine until I tried to import them into macOS Mail. Mail wants a mailbox format (.mbox I believe), but I only have .eml files. It let me select all of the .eml files and import them anyway but it's taking about 10 seconds or more per email and I have like ~17,000 emails. ChatGPT says thats 47 hours 13 minutes 20 seconds. It's also freezing the mail app during this insane import process. It would be great if I didn't have to wait that long and that's assuming something doesn't go wrong/the app crashes.

No, I don't need all of those emails (I actually deleted about 10,000 over the past month) but now I'm kind of screwed. I guess for security reasons, when you delete the domain from ProtonMail, it deletes all the emails (as far as I can tell, at least). I'm mentioning this because I was thinking I could have tried to find a way to export it in .mbox format or even delete more emails but it's too late for that.

The way the emails were exported, the filenames are jumbles of letters and numbers so I cant even use Finder to search for specific emails when I need them. I can open each individual email in Mail, so it's not like the files are encrypted, I don't think.

One thing I thought of -- I once used Mail with ProtonBridge that would allow you to use ProtonMail with the Mail app, but still use Proton servers and it synced all my emails with my computer. I disabled that account some time ago (didn't delete it or remove it completely from the list of accounts, just unchecked it). Do you think that directory of emails is still on my computer somewhere? And would it be useable in my situation?

I do realize this is completely of my own doing and should have been more thoughtful but I'm here now and would love any help y'all could give. Thanks everyone!

Edit_1: Oh no... I found the directory for Mail in Application Support and it looks like the Mail importer is making a mailbox for every single email. I tried importing these emails earlier today and it looks like it might have imported everything? Here's a screenshot. 🪦

I wonder if I could try and import them into another app and then export in a better format? Thunderbird? Ugh, I really do need a lot of these emails…

Edit_2: So the export tool mentioned above has a restore feature so currently trying that. It will just import these emails back into ProtonMail and I’ll have to figure out the export part again. Hopefully this works!