Noone is saying, he can't have it included in his sales, nor is his BST only limited to having "pagebuilders" sites.
He could advertise as building a wordpress website, and scale it up from pagebuilders to oxygen to custom CMS.
That is still gaining the market of the 1% of the forum.
Scaling is not working that way. If you are offering so many services, you need a team.
Really, he won't be able to handle the workload solo. He must make the choice to what offer and to whom offer.
You can build affiliate themes, multipurpose themes, single-use themes.
You can extend pagebuilders, you can use frameworks, you can use plain PHP.
You can write for sales purposes, for SEO, for documentation, for books.
The real problem is OP doesn't even know what he can do.
All I can advise is to do market anylysis and focus on selling one thing to as many people as you can reach for the cheapest cost. Calculate this.
Also, pick the thing you are the most comfortable with, so you minimize the effort.
Then build your team.
If you have to offer 20 different services and have no team, you're doing something wrong...
First, make the choice. Bespoke services or template websites. Then think about your skills and the best fit.