We added a couple floating shelves ourselves.
These are about 1 3/4" solid pine. I glued and bolted them to 1/2" plywood and also added hardwood behind the plywood to strengthen everything. I cut out the drywall and screwed shelves/plywood to wall. Tiled around shelves over plywood.
The whole assembly is very strong.

Thank you for describing how you installed your floating shelves. I'm looking at doing that and this is useful information!