FileVault 2 Full-Disk Encryption Alternative with LDAP Support
Nope - there’s no getting around FileVault at the moment and with the T2 chips controlling this, you’ll need Apple to move to get the alternatives you mention off the ground let alone to production.

There are very good pre-log in screen options like the NoMad family and JAMF Connect that allow you to have LDAP/AD users checked at log in time, creating a local user if needed so you don’t have to pre-set up local users on macOS.

You’ll need an MDM to collect your FileVault keys (individual or institutional) if you need to be able to unlock a mac where a user isn’t already FileVault enabled. You’ll also need to keep track of secure tokens on newer OS.

Hit up the mac admin community on slack if you manage macOS professionally - most of the tooling needed is open source and the community blogs to get you up to speed are invaluable.

You’re right this isn’t manageable with tools like Apple Business Manager / DEP / MDM so you’ll need to bring some tools to manage 500+ Macs without a large staff and tons of repetitive work.