Is there a word for ....

a hatchling who inherited every gene in the combo? I feel like there ought to be. Yahtzee? Bingo? A "royal" flush maybe? Overachiever?

Not that I know of. I just always hear people say "I hit every possible morph" or general terms like jackpot.

Oh "jackpot" is good too. It just seems like one of those things there ought to be a name for, doesn't it?

Yeah, probably should be!

Typically the phrasing I see people use are:
The all gene animal
The jackpot
The jewel
The star


Or if it was me ... "My holdback"
Although the all genes in my clutches this year will be interesting to see or absolutely impossible to see with BEL complexes involved. Lol

The phrase "hitting the odds" may apply in this circumstance. Breeders say that when it is a long shot, or harder to produce a certain combo, but I would imagine that in some cases this may also apply to what you are referencing here.

Hitting or smashing the odds.
The odds gods are pleased.

But that's like, a phrase blanketing the animal. Not something you would call the individual snake.

"We smashed the odds on this one. Here's the Jackpot" sorta thing.
One is about the conditions, the other is the snake