Looters like Diablo and Borderlands?

To be fair, these aren't the words I'd use to any game you've mentioned. Something like Doom and its sequel, perhaps.

Or take just about any single player game and play on easy. Boom, there you go.

You want to put low effort into complex systems and get high rewards?  What?

Basically any modern game. Most games released now are easier and more accessible.

A low effort high reward game would be like Rocket League and Counterstrike. The concept is simple, you have basic objectives but the skill required to get ranks needs training and perseverance. Any scrub can jump in and get Silver but to get to gold, elite and global you need a strong team or skilled players who know what they are doing and be consistent.

I would say the Souls games if you are talking SP you going high reward. It takes time to master but the concept is simple. WoW is another good one, easy to pick up and play but PvP and endgame requires you to use your brain (and skills).

Sea of Thieves? At its core it’s a low effort game where you make your own tales to tell. Ends up being one of the more addicting multiplayer games of Xbox One

This thread makes no sense. As others have said, you can take just about an good game, set it to low difficulty, and you have a game that matches your requirements. Some of the games you listed yourself in the OP are a great example of this, in fact.

Give Divinity Original Sin 1&2 to just about anyone on Tactician mode, and their first playthrough is NOT going to be low effort. Or you could be really cruel, and set their first playthrough to Honor mode. Get back to me with how little effort they're putting in, and getting by.

What's complex about TW3? Or rather how is it different from other open world RPGs ?

Low effort high reward, that's Mario games. At least when you say that I think of easy to pick up difficult to master.

I disagree with almost everything you've said here.

For starters, how does one define what is and isn't rewarding to the player? For some, it may be discovering new places on a map. It may be getting a powerful new item. Completing a story quest. Leveling up. Getting to the next cutscene. Experimenting with game mechanics. Meeting, and befriending new characters. Taking screenshots in a beautiful environment. I could go on all day. Literally any number of things may be rewarding to any number of people, so if you're going to make the general assertion that some of these games aren't rewarding (be it in short play sessions or otherwise), I'm here to tell you that you're wrong.

Filled in one critical thing that was missing

You go ahead and do that. It has the best combat in the entire franchise, so that alone might be plenty rewarding for you.