converting SQL . BAK to SQL backpac format
They are different formats. "bacpac" files do not include Transaction logs and are not transitionally consistent by themselves. You can read more about it here "Windows Azure SQL Database Backup and Restore Strategy"
Your best path would be to restore the backup (".bak") of the database to a local server, and then to create a ".bacpac" file from the restored DB. You don't need the restored local copy of the DB to be high performance (other than for the purposes of extracting its data into the bacpac) so you can restore it onto a server that just has enough space to accommodate the data. That should help with the "the DB being huge for my infra" problem.