Room acoustics is a black art. Sound travels differently depending on hard/soft surfaces, reflections etc. Even moving the Sub a few inches may make a difference. 

Yeah I think I should try some more positions, but my partner needs to agree with the position as well. 

The listening area is around 25 meters square, (photo added, taken at couch distance with a wide angle lens, there’s around 3 meters to the back and it’s 3 meters to the wall in front.) but there’s no wall to the kitchen, making the total area 37 meters square. It’s not really open concept, because the kitchen starts at the end of the left wall in the picture. 

The TV can’t be moved, my partner won’t allow it, because it would cause the couch to be smack bang in the middle of the room, leaving almost no usable room for the kids. 

I added another picture for a broader overview of the listening area as seen from the kitchen. Would I have been better if with a sub gen 3? I could still exchange if I have to for 14 days. Yes it’s cluttered in the pictures; taken before really setting everything up. But it should get the room layout across. 
 

Moderator Note: Removed content as requested by OP.

It’s quite a large area. Although it’s in the “Sub Mini with Arc” section, the FAQ’s for Sub Mini do say:

 “If you have a larger space or intend to listen at higher volume levels, Sub will provide a better experience.”  

Depending on your funds and time left to return the Mini, buy the full-size Sub, try it and compare, and return the unwanted item? 

Simply replacing MINI with SUB will not change the room characteristics. Your preferred subwoofer location interacts poorly with the room. Unfortunately, you cannot change the laws of physics. Moving a subwoofer a few inches can result in a large change. Moving the listener makes a difference too.

This is my room layout. I can’t really change much about the lay-out. The line between living room and kitchen is not a wall, it’s a measurement line in the drawing. So there’s no wall between living and kitchen. 

Red dots are possible positions with approval and electrical outlets. Right now it’s at number 2 and the only spot I can actually notice a difference between sub on and off in the room is at the dinner table. As if all the air is pushed diagonally towards the kitchen instead of towards the couch. 
Spot 1 cause booming bass that’s unpleasant to listen too, you can also easily notice it’s coming from the right. Spot 3 on the left sounds a tiny bit better but I still have to EQ bass to +2 and have sub volume on +5 to notice a difference. 

Spot 3 on the right, haven’t been able to try out. I don’t have a lot of time this week to experiment sadly. But the music and video experience is pretty underwhelming right now, or overwhelming at spot 1 in a bad sense. 

I know a bigger sub won’t change characteristics, but I also don’t really understand how the bass travels all the way to the kitchen (and even a bit BEHIND the couch). But isn’t really noticeable at the one spot we want it to be noticeable. 

You can explore the physics by playing a track with thumping bass, then walking the room, close to the walls, while carrying a pile of Post-Its and a magic marker. As you encounter a spot with enhanced bass, place a Post-It. Certain locations, such as #1, will have doubly enhanced bass. Mark these Post-It’s with !!. The Post-It’s are at “peaks”. Approximately midway between two peaks, is a “null” where the bass will be minimized. The effect is symmetric. If you place a subwoofer at at peak, it will be more efficient. As you become familiar with the process you’ll be able to find smaller peaks and nulls in three dimensions throughout the space. I suspect that your current subwoofer location and listening position are at nulls.

You can also turn the walk inside out by placing the subwoofer at a proposed listening position. After the walk, you can move the subwoofer to an appropriate spot, since you now know the locations of peaks and nulls, relative to your listening position.

In my college apartment I found a monster peak and placed a chair on it. This was the favorite spot for males because the bass seemed to explode from within their body. If I moved the chair a few inches the effect was gone.

Tone controls can be constructive. They can amend the system response to accommodate room conditions and user preferences. I know a fellow who always sets tone controls at max and places SUB at a location similar to your #1. This sounds “best” to him. Fortunately for me, I don’t need spend any time in that room because it sounds very “bad” to me. I don’t need to comment to him how offensive the room sounds to me. In fact, I helped him setup the room, knowing his preferences, and I’m a hero.

Trueplay can be helpful, but it will not result in “best” for everyone and some room situations are beyond Trueplay’s ability to correct. Trueplay would not be appreciated by the fellow above.

So I replaced the sub mini with the gen 3. And I don’t get any bass at all still, +15 for it to be noticeable, even in the corner :(

Have you tried to call Sonos Support directly to discuss it?

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.