So, your existing devices are AD joined. You should set up Entra ID Connect if you haven’t already and turn on hybrid joining. https://learn.microsoft.com/en-us/entra/identity/devices/how-to-hybrid-join This will get your devices registered into Entra.
The next step is to set up the GPO to join the existing devices to Intune silently. https://learn.microsoft.com/en-us/windows/client-management/enroll-a-windows-10-device-automatically-using-group-policy
You can then upload the Autopilot hashes automatically by enabling “Convert all targeted devices to Autopilot” in your AP deployment profile. https://learn.microsoft.com/en-us/autopilot/profiles#create-an-autopilot-deployment-profile I wouldn’t recommend bothering with making a project of wiping and Entra Joining existing devices; Just let existing devices age out and move to Autopilot and EJ for new devices.
For brand-new devices, you should be getting your hardware vendor to upload the AP hashes for you at order time. If they can’t/won’t, or if you already have a pile of existing devices, you can use the registration script during OOBE to upload the hash. (skip to step 2, I’m not even sure what step 1 is for) https://learn.microsoft.com/en-us/autopilot/add-devices#powershell
This can also be made dead simple with a provisioning package (literally just plug the USB in and wait) https://www.modernendpoint.com/managed/Silently-Collect-Autopilot-Hashes-using-Microsoft-Graph-and-a-Provisioning-Package/
Also remember that Hybrid Joining during Autopilot is almost never necessary, makes your life a living hell, and should be avoided at all costs. https://wiki.winadmins.io/en/autopilot/hybrid-join-vs-aad-join
As for imaging, if you’re getting your devices from an enterprise vendor, they SHOULD be giving you a pretty clean image, so it shouldn’t be necessary. Note that if you’re moving to Windows 11, MDT is not technically supported. We’ll just be using the provided image from Dell when we go to Autopilot and Win11 next year, but at the moment we’re still using MDT for Win10. You could look into something like OSDCloud to build your clean images if you still choose to do so.