Licensing

Hello all!

I have read much about the GPL license in general, but it is not really clear to me if the following scenario would be possible / allowed:

In general we want to develop a WP theme, that will be hosted on WP instances on our server. Customers will have to pay a monthly fee for the complete solution and they has the possibility to buy-out the theme to host it on an own webserver. If a customer buys-out the theme, he won’t be allowed to redistribute the theme to others. (like bought themes from different theme-marketplaces)

Would all of these will be possible / allowed through the WP GPL license?
In GPL the copyleft confuses me a bit… (sure that the theme needs to be GPL)

Thanks in advance!
Regards