I only ever used it for 3D printing but FreeCAD apparantly also has a floor plan functionality. I found this video that might give you some insights on how it looks: https://www.youtube.com/watch?v=WMk7pdnEnXM

When using FreeCAD for 3D printing, you don't necessarily have to add measures/constraints to everything, so I'd assume it's similar here.

LibreCAD is 2D CAD.

I always use gimp. Just define one pixel equals one millimeter, and then make all the furnitures as separate layers so you can move them around separately. Works great!

I've used illustrator but ink scape (free) would also work. I just use basic shapes and just ignore the unit. Meaning that I consider a mm in the software to be an inch IRL. That way I can get everything into proportion in relation to each other. I use the text tool for labelling furniture and stuff, grouping, and align tools too. It's helped me tremendously.

if you use sweethome and only look at the top view its as good as a 2d application.

I feel like youre just being fussy here man

What's wrong with adding constraint dimensions?

I tried draw.io for a similar task and, while it worked alright, I agree that it's a bit too much work to set up nicely

There are also some CAD tools that are FOSS. Maybe some of those can help?

Wait, sweet home 3D was sold? Is it not open source yet?