Did you consider this: Natural Earth II is a raster map dataset that portrays the world environment in an idealized manner with little human influence? Can be combined with the regular vector data form Natural Earth.
The QuickMapServices plugin actually has a few base layers without labels.
Dark Matter [no labels]
Dark Matter [no labels] (retina)
Positron [no labels]
Positron [no labels] (retina)
You might need to install the "contributed pack" to get access to the maps listed above. Note that this setting will ...
Indeed it is a challenge to find free basemaps without labels.
I can provide this list of no-label basemaps:
Carto Dark No Labels:
Carto Light No Labels:
Carto Voyager No Labels: