Pre-select blog template or hidden field?

Hi,
how can I pre-select blog templates for different registration pages?
here is the situation:
On my network I sell sites to unrelated categories like (fitness, lawyers, dentists...) through different sales funnels which eventually needs to end up on different registration pages.
For each one of these categories I have created a related optimized blog template.
How can I pre-select for examples blog template1 for sales funnel1, template 2 for registration page 2 and so on? I don't want them to see the option since my clients need to percieve the network is only for their own category.
I was thinking of using some hidden field... but I am not really sure of what I am talking about....( I am certainly not a coding genius)
I really tried everything, even whmcs but can't get close to the solution, even though I feel "new blog Templates" is the way to go....
I also considered this. https://premium.wpmudev.org/forums/topic/new-blog-templates-12-with-multi-domains-integration
but I don't need other domains really...
Thank you