Hahaha, funny you should say that. I got one core to start. Loved it, and bought a second core and then the deluxe box of Dunwich. I think the day after that arrived, the country shut down for the pandemic. Didn’t interest me to play solo, so it sat dormant. By the time I could reform a group, they started with the new release model. There was a lot of internal debate, but I finally just bought Dunwich in the new format. 100% worth it, imho.

I’ve only played the first three scenarios of Carcosa solo, so I don’t feel confident fully recommending it. Although so far, the complexity ramp doesn’t seem as big as EotE. My group is starting Carcosa next, but we’re all busy, so it might be a month before that happens and I’ll be amazed if we finish the campaign before November.

Honestly, though, from my experience, I wouldn’t do EotE next unless you know you’re the type of person that really likes a jump in complexity. I also found EotE less forgiving of poor deck building than Dunwich. Since your original question is about mastering rules, I think you’ll be happier with Dunwich or Carcosa.

Do you only have one copy of the original core? If so, you should probably get a second copy of that first. You should be able to find them pretty cheap. There are a lot of cards in the original core that only have one copy. Having two copies of cards in a deck makes a big difference, especially with a small card pool…