Alright, I am officially a fool X.x All the elaborate workarounds why it is SO incredibly easy...
I got to thinking, TS4 is basically designed to build, not to remove, so I figured this might work:
1) build a "hall" around the stairs and remove ALL the floor (now you have a walled room without floor)
https://i.imgur.com/DlV7ENy.png
2) build the floor you do want in the hall, so basically construct the floor around the hole. Use a platform and
lower the platform to normal room floor and feel free to draw the arrows around to fit your need for the opening. (You can also use a deck instead of platform, no need for lowering) You can also keep adding more pieces of 'floor' (diagonal corners, pieces you want to cut out etc.)
https://i.imgur.com/EGgIAUh.png
https://i.imgur.com/oVNSxQa.png
3) Don't forget to add the floor back above the stairs you are placing from the floor below! If you don't place these 3 blocks, the stairs have no 'ceiling to remove' and the staircase is not functional. (Honestly, this is new to me since my last workaround, never noticed this before) EDIT (14/05/2021): they fixed this in the last patch, no need to readd the ceiling above the staircase! Edit(28/05/2021): last patch broke it again, might want to just keep doing this to avoid future problems.
https://i.imgur.com/YzOJHOY.png
4) Finish off however you like, half walls, fences, nothing, debug stuff.
Keep in mind, once you do place a wall or fences, if you remove them you 'explode' the room (no floor + removing walls constitutes not a room) So either remove them by dragging the room out 1 line everywhere, adding platform over the fence (both sides, then it disappears) and then lowering the platform/redragging the hole to original shape or start over, it really doesn't take long.