Here is the modlist i used on my last Ironman playthrough. It is very lightweight, with only a few carefully selected mods instead of a giant list of random stuff. Was a blast, this worked extremely well for me, so i wanted to share.

It is built around A Better ADVENT and Raidey's Class mod. One extends and improves the aliens, the other one overhauls the XCOM classes. Together, they already do almost everything that i want.

Here's a breakdown of the mods:
- The two big ones -
A Better ADVENT: As the creator of the mod, i am obviously also using it. Duh.
Raidey's Class Mod: I like that this mod doesn't introduce all kinds of overpowered and/or overengineered new abilities. Instead it used vanilla assets and reconfigures them into 8 new classes that are balanced against each other. I am really impressed by this mod and how much it shakes up the game. Doesn't get near as much recognition as it would deserve.

- Tweaks to gameplay and difficulty -
Better Pods: This plugin for Better ADVENT improves many aspects of pod composition.
Fixed Squad Size: I don't like the mods that increase the own squad size very much. Turns start taking forever clicking through all of that. It also gets unbalanced really fast. FSS gives a boost to the early game while still keeping the late game interesting. I like.
Scanning Sites Plus: Improves the choice on the tactical map by making more scan sites spawn and giving you different options on what to go for. Many of the scan sites generate extra missions which can make the campaign almost feel like the old Marathon mode. The mod comes with an ini for adjustments, i went with 4 max scan sites and a 20% chance for a mission.
Explosions destroy corpses: A well deserved nerf to Grenade spam.
Extended Dark Events: I disabled the permanent ones, but the others are great. I was hit by the Squadsight event for a month... 10/10, would lose two soldiers to a Sniper Captain again.
Quick Start: Because Gatecrasher is nonsense.

- Quality of Life -
Gotcha: Probably the most important mod on the Workshop. There is not a single XCOM setup that doesn't want this mod.
Additional Icons: Now this one is almost mandatory for me, because it adds nametags to the aliens. With Better ADVENT, this is immensely convenient. I did however disable the Loot Icon, that is just straight up cheating and too much of a buff for grenades.
Perfect Information: Not as necessary, but i like knowing the odds that my overwatch shots and enemy fire have.
Blackmarket Usage, Mission Time Remaining, Upcoming Events In Hours: These three display some stuff that i want to see more clearly. Not vital, but convenient.

- Other -
I didn't list the cosmetic things. I use Capnbubs' Accessories, the Advent Armors and the Total Advent Weaponry. Oh, and Additional Mission Names because vanilla names seem to repeat a lot.