How can I fully bleach a polyester shirt?
Bleaching works "as expected" for natural plant fibers (e.g. cotton). Polyester is actually a plastic, and bleaching will not have the same effect. I expect one (some?) of the following:


no change;
some fading, potentially not uniform;
(partial) destruction of the shirt, as a result of the bleach attacking the plastics. Even worse, if there is anything natural hidden somewhere (some stitching, logos...)


Bottom line, I would not even try. A new white shirt must be quite easy to find, and quite cheap.