Dang. Some information I've been pointed to since publishing this suggests that there are multiple groups out there that consider it likely that photosynthesis was present very close to the root of the bacterial tree, and that large numbers of bacterial groups may have lost it rather than it going all around the tree by horizonal transfer. This would put photosynthesis as one of the rather earlier metabolic pathways on Earth.

I've also been pointed towards more modern evidence that oxygenic photosynthesis originated in the original bacterium that created both energy-producing and sulfer-creating photosynthesis rather than the two coming together via horizontal transfer later.