Since your assets are not in ABM, you can issue an unenroll to drop them from Profile Manager (or go to the device and delete the enrollment profile as you cannot lock a user-initiated enrollment). Then, use the user-initiated enrollment URL of your new MDM (for Jamf Pro, it is something like yourhost.jamfcloud.com/enroll). Please note, performing a user-initiated enrollment will not achieve full supervision as a DEP enrollment is required for that. And if you had profiles whitelisting system/kernel extensions, PPPC rights, etc., you might get a batch of prompts was the removal of the enrollment profile will also remove all of your other profiles.
If you are returning some of these devices to service, and they are eligible for side loading, you can erase the device and use Apple Configurator to inject them into ABM. This requires a full device wipe and is limited to T2 Macs or higher and all iOS compatible iPhones and iPads. Do this ONLY if the device is idle and being reassigned.
Now, the other obvious recommendation is to replace your hardware. If you have gear in Profile Manager, I will bet the equipment is at least 4 years old (Server.app was dropped in 2022 and last "ran" on Monterey - but the decline began many years before). The Macs are likely all x86 based. If so, consider doing a fleet refresh. This will allow you to purchase the equipment properly, allowing it to be associated with ABM. This will allow you to setup your new MDM for a full automated enrollment process. Replace the units and migrate only user data.
As for VPP content, if you need a period of overlap, simply log into ABM and create a new location. Then license/purchase additional apps and assign them to the new location. This new location will create a new VPP token, allowing you to bridge the migration from Profile Manager to your new MDM. In otherwords, you will not be revoking VPP licenses on existing devices.
Hope this is helpful. Run, don't walk, away from Profile Manager.