When would you buy a Wordpress Theme over using a builder, or have a website made from scratch?

It's great you're reflecting on data and results to move your business forward. Personally I find the options that exist now for website building are plentiful that it's pretty easy for folks to self-serve, even for more complex types of sites. It may be helpful for you to spend some time speaking to business owners who use Wordpress to understand what kind of support they would need and offer some support there?

I think is really depends on the individual business context and model. Far more niche and complex websites/apps will almost always need to be made from scratch. However, everything from a simple static site to a complex marketplace can easily be created using many readily available self-service tools like @abadesi mentions. I've seen very interesting setups for eCommerce marketplaces using only Wordpress and a plethora of very cool plugins to get what they want. Low code solutions (like Wordpress) really seems like the direction the industry is moving to, and if people with literally no coding experience can use it, it's likely going to become hugely successful. Don't forget that from a psychological perspective also, we value things more if we actually make them ourselves, hence why self-service website makers and easily customisable templates are hugely popular. Perhaps the solution to your problem @yakitos is a hybrid between making templates and your current set up? I'm thinking the core of your business would be focused on template creation and then you offer a side gig where you set up, customise and host your client's website, using the template they just purchased!?