Thank you so much for such an encouraging comment!
We're planning to update the game ASAP (ETA today or tomorrow) to include some sort of tutorial and to add an ability to backtrack through any node you've beaten (at least for now, we will probably change how it works in the future).
Long term we too want to expand on monsters roster and add multiple "dungeon floors" where you progress deeper and deeper with the same deck, akin to other dungeon-crawling deck-builders.
About numbers on the bottom left. On top you see your current "energy" which you spend in the shop, just under it you see five numbers. The central one, as you observed, is the number of cards, and four numbers around it are your card connections count (so the top one is the number of cards with the connection on top). We will cover it in the tutorial, but in our internal testing we found that it's essential for deck building to have balanced connections count.