First, thanks for your reply to my questions. I need to read the 5e DM's book more. I have sooo little time.
As to The Expert rules. I played with those, back in the day... (I'm almost 41 YO now). I used those extensively in the adventures/ campaigna I created then, though I don't remember much, except the players got lost... a lot... I'll need to reread those. (Sigh... I have no idea when).

As to your comments regarding the last session, with the meanlock prison:
- It's very hard to create a small scope set piece (such as an encounter or an NPC) to work as imagined (even the boss who turned into a snake in your first adventure). Oddly enough, it's easier to invoke a certain feel from an adventure, an arch or campaign, because those have multiple opportunities, and you can adjust and reshape concepts.
- Another thing I notice you talk about (possibly without referring to it as such) is letting go of control as a DM. D&D puts a lot of the onus of the game on the DM, and as such many DMs feel pressured, and try to control how the game will run and more. When I came back to DMing after a long hiatus some years ago, I implemented a rule where "players roll all the dice". I originally did so in order to reduce the number of dice roll and math I had to do, and enable me to keep my head on running the situation and scene, but it had an utterly unexpected result- by making all dice rolls utterly open, and telling the DCs upfront, I wasn't able to fudge. (And though I lied to myself previously telling me I fudged only very rarely, I quickly became how much I did fudge... the lies we tell ourselves). And... trivial stuff became hard, hard stuff became easy, and more...
Many unexpected things happened.
At first I was very much alarmed, and partly panicked- I was losing control! I will I deal with what happend? But... to my suprise, I also enjoyed it immensely, and it made me think and prepare my adventures differently, better. It made me a better DM, made the game a WHOLE lot better, and proved a great deal of fun!

It was a great lesson to me. And it seems to you as well?