I have an answer based upon my experience, but it’s not the only answer.
Here’s my approach:
1. Drop Zapier and use Make (Integromat) instead.
They have more power and flexibility, especially for what you’re asking about.
They have an ample free tier and when you’ve outgrown it you need to spend $9/mo versus $29/mo.
2. Drop Sendinblue and use ConvertKit.
Again, they have loads more power when you need it, but keep it simple when you don’t. They specialize (very well) in nurturing your subscribers over the long term. That’s where the profits really come from and this tool gives you that.
They are free to get started (just like Sendinblue) but when you need more power it’s there and doesn’t cost a fortune.
3. User Webflow Memberships.
Gate everything with Webflow Memberships and pay for just a basic account.
This is really the only subscription you need to pay until your masterclass students are paying you.