AllTheMods has plenty of progression options that are reasonably casual but also quite deeply technical, at least depending on what path you choose to pursue. They tend not to be the kind of modpacks that force you into too narrow of a progression path, so as long as you can trust yourself to intentionally explore some new mods instead of falling back on familiar patterns, they're generally a very friendly and well-made experience (and because they're AllTheMods, you'll probably also have a wide selection of comfortable, familiar mods to fall back on if you get stuck with some aspect)

Without knowing too much about what you like and what you've tried before, that would be my recommendation. I can try to suggest some others if you want to go a bit deeper into what you're looking for. How deeply have you played around with Create?

Have you looked at Create? Disclaimer: I've never actually played it myself so I'm not sure if it's exactly what you're looking for

I gave GT a go playing Nomifactory and it was a ton of fun actually. Quite an old MC version now but it holds up.

It's designed to be played on Peaceful, just you and your machines, and it has the lost cities world spawn so the vibes were really chill.

I'm currently playing through Star Technology which is similar but on a skyblock. Both are a bit grindy to start with but very satisfying once you get an ME system going.

the Enigmatica expert packs might be of interest to you. a bit older, buit still good to play.