Freeware for floor planning - recommendations for a tool?

I can only recommend taking a look at SketchUp, as it makes you more flexible in the long run. For me, the projects in SweetHome3D eventually became too large (I had also modeled the neighboring buildings to get sightlines to our house) and some constructions were hardly feasible (e.g., internal shed roofs, terrain slopes, light courts, ...). SketchUp is much more flexible in that regard, but it also takes some time to get into it because you have to understand the philosophy of the modeler. Drawing walls, windows, doors, etc., works a bit "differently" there. In my opinion, the effort to learn it is worth it.

However, you can also get quite far with SweetHome3D. It's a bit a question of personal standards. A little tip for SweetHome3D users: There are really many useful 3D models in SketchUp's 3D Warehouse (Ikea furniture and so on..) that can also be exported into the object format. This way, you can get them into SweetHome3D as well. It's a bit fiddly but doable.

However, the renderer of SweetHome3D is terribly bad and inefficient; that was also a reason for me to switch because the time required for sun/shadow simulation eventually annoyed me a lot. But to quickly put something together and get an impression, it’s definitely good enough.

Maybe asking a bit much, but: Is it even possible with SketchUp to simulate sunlight at specific times of the day (preferably depending on the seasons) that also includes neighboring buildings?

And can you also do walkthroughs of the object as photos and videos with it, and accordingly faster and better, as I understand it?

I don't remember off the top of my head right now. However, it should be possible with animations. Just Google it [emoji6]