Believe it or not, I am right there with you on this. There is little to no way that I can see to do this without leaving most plants facing more competition than they can possibly cope with. Maybe start with islands that already have low plant biodiversity, and try to splice as many of each variety of plant there as possible at once? … but that’s all I’ve got.

Spliced Oceanic Algeas would probably just fill the oceans to the point of killing everything else in them.