Fixed it by doing the following - there were two errors.
First, I fixed the second error by doing the following:
rails c
Upload.find(386).destroy!
exit
This fixed the corrupt post(malformatted text or something)
Then the red errors on the screen were from using the random subdomain(provided by cloudflare) and uploading to that. I had to upload the files to the new domain(which includes my connected domain)
Using:
sudo -E -u discourse bundle exec rake s3:upload_assets
Then:
sudo -E -u discourse bundle exec rake uploads:migrate_to_s3
Then the forum properly loaded and right clicking and opening images in a new tab shows they are hosted on the R2. Then I did a backup.
Finally, I cleaned everything up using:
sudo -E -u discourse bundle exec rake uploads:clean_up
Which freed the respective storage space of the files(note I still have a local backup of both files(OS level) and discourse level before migration.
Now everything works perfectly!