I still suggest to copy the personalizations from another user. This is the easiest approach. You could even have some dummy/template user for this, and keep it disabled. In this case the user would not cause any license cost as far as I understand.
If it's not feasible, then you need to break your problem into smaller ones.
1. Import personalization file in batch
2. Apply personalization file to user in x++
For the first one, just search the web for how to import files programmatically to D365FO.
For the second one, I think you need to study how the standard, form based functionality is implemented, and then try to implement similar solution without having dependency to the form.
Also in general, if you want to apply some form customization to all users that are created, I suggest to rather change the forms via development! It could be even cheaper than building all these integrations (of course depending on the complexity and amount of personalizations).