The email export and migration is the hardest part. There may be some paid tools that could help facilities it into a major email provider.
If the accounts are now via office365 then you can use the outlook desktop client and export your mailbox to pst. Your account may also allow connecting by powershell and exporting to pst.
Thunderbird may be able to cache it all as well if somehow imap may still be allowed.
Once you know your exports are good then you can proceed with the below.
Switch dns to cloudflare free.
Pay for new email provider. Setup your accounts. Import the pst data.
Set your mx (mail records) to new email provider.
Do a updated export/sync of your godaddy mailbox after 72 hours and import the delta into your new email provider.
If you're non-technical and don't know any of the above then ask a friend or pay for some help.
Your extensive config is probably your issue and not opnsense. You said you've been running it for a few years but seemingly 4 months ago, you couldn't figure out a basic rule to block internet for a single ip.