I like the idea in that other post.
I have a couple ideas on how you might be able to grow the GP business model. Like I mentioned I personally use Ninja Forms and I like how all of their add-ons are individual so you can pick and choose what you need. Each add-on also has 1, 5 or 20 user price tiers. Their individual add-ons are each about the price of your entire GP premium. This might be another thing to think about, possibly raising the single GP add-on costs and/or creating multi-user/annual licenses instead of unlimited like it is now.
Another thing I was thinking of is I see a lot of theme groups using the same plugins on all of their themes. The same slider, contact form, pagebuilder, etc. What if you were to form some kind of partnership with your preferred plugin developers for things like contact forms, pagebuilders, caching plugins, etc. and create premium bundles that come with GP. For example I pay for memberships with ninja forms, wp rocket, etc. The sales pitch could be that these are trusted plugins that work great with GP, why pay for each separately when you can bundle and save. Then you could include installs of these premium plugins with an install of GP and have an annual renewal cost.