Thunderbird doesn’t understand aliases by default (apple’s mail apps on MacOS and iOS do). You’d need to add the alias under Account Settings -> Manage Identities for each alias (which is any custom email domain accounts you add, assuming you want to send mail as that user). There is only one account: the iCloud login. Everything else is treated as an alias, and doesn’t create its own inbox - everything goes into the singular inbox.
As the other posts said, email won’t migrate automatically. The easy way to do it, though, is setup your old email and the iCloud email in email and just drag and drop your email from the old email to your new iCloud one.
Keep in mind that you’re going to be retrieving and storing a huge amount of data running these scripts, and you should expect to need more than a $5 1gb of RAM vps to do it without it being a shitty broken experience for you.
We’re talking dozens of gigabytes of storage for the database, plus effectively a need for an infinite amount of storage for the image caching, plus enough RAM and CPU resources to effectively process the whole Threadiverse.