As a separate reply, I'll add that I think finding where/if this breaks is pretty academic.
Eg: you wouldn't build a glider out of heavy material that gives you huge speeds but also huge sink rates.
So I think the entire glide ratio conversation mostly fits in the "your plane is fully loaded" vs "your plane is empty" scenario, and the point is that your best glide ratio will be constant, but you'll be gliding at higher speeds if you have more weight.
Eg: you wouldn't build a glider out of heavy material that gives you huge speeds but also huge sink rates.
So I think the entire glide ratio conversation mostly fits in the "your plane is fully loaded" vs "your plane is empty" scenario, and the point is that your best glide ratio will be constant, but you'll be gliding at higher speeds if you have more weight.