Two things.

1) I think you should drop the blank aspect. There are a couple of reasons, but the lore one would be that research into blanks simply wouldn’t have developed enough for a successful or stable attempt at gene modification before the project was shut down. Genetically modification to create blanks is necron level tech.

2) What is the balance of the Avvim? Because if you look at abhuman species in setting, while they are better than baseline humanity at certain things, it comes at the cost of something else. Ogryn and Golith are both big, strong and tough, but at the cost of being less dexterous and intelligent. The more you tailor a species towards something, the less able it is somewhere else.