I have looked at software like Sweet Home 3D, OpenPlan3D, and various cloud-based services that cost money. As nice as they look, they each have severe limitations – main the inability to create accurate measurements for walls. For example, Sweet Home 3D doesn’t give enough precision to create a wall with a 9/16 fraction. OpenPlan3D the field is disabled.
I would like to be able to build a 2D floor plan, but also be able to convert it into a 3D model that I can then add furniture to for interior decorating. I would prefer open source, but would settle for simply free to use.

This can be done in Blender.
Here’s a new addon specifically for floor plans, although it’s not needed. I haven’t used it, I think there are others.
Blender won’t be as easy to use as some purpose built app, but it can do anything you want. There’s also a huge library of pre-made furniture models in Blenderkit, which has a free and paid version.
Thank you. That looks very nice and viable. Bonus that it gives me an excuse to learn blender.