Maybe I am misunderstanding but I don't think this is true. Indeed you pay for the inbox, but it's not the primary domain. You can reply from any of the email addresses you created (which doesn't cost extra) and it automatically sets the FROM to be the email it was received on
If you have Fastmail configured to send from that domain as well, it will automatically reply from the same address the email was sent to. You can pick which address to send from, of those you have set up.
I think this is pretty interesting, especially if you could remix courses... like if you had a bread baking playlist and a pizza making playlist. The pizza making course could include or use modules from the bread course as a requirement.
Similar with learning software development, you usually need basic modules like regex or databases before learning more complex skills.
Looks great, but I find the solid line under the navbar that sticks to your mouse a bit annoying... as it covers and crosses out the logo when you want to press it, you could also get rid of the sticky nav because the page isn't too long. The extra bold black line kind of clashes with the otherwise light and airy design you have around the rest of the website (but looks great in the footer) so I would say use it in moderation. Good luck!
In one of the YC videos I think they called that a "solution in search of a problem". A couple years ago everything was blockchain technology searching for a problem, then it was VR technology searching for a problem, and before that it was social apps... but if you start with a problem first it's easier to build the right solution and skip the latest hype cycle.
I think AI is hyped up a lot right now and there's definitely going to be a trough of disillusionment next or another AI winter, especially as all these AI startups fail to find a successful business model.
If you are worried about someone replicating the idea with AI easily then you can spend some time focusing on your business moat, which AI companies are struggling with now, but especially with cloud security or cyber security I would be worried about trusting anything that was vibe coded or had AI integrated because it's a black box.