"World is literally infinite." but not so infinite as to encompass demi-angel tigers, it seems :P well, the brain thing is disappointing :( couldn't the angels magically tether their psychology to unfitting brains, or something like that? Give the embryos special magical protection via spells or potions? They're immortal, I can't imagine no-one has found a workaround yet.... well anyway, if I suddenly woke up as an angel in your world one day, I'd know what my next project would be. :P
"World is literally infinite."+It's a discworld without edges.+There's space for anything that is local-physics-compliant.+Also, weird geography and climates.
"couldn't the angels magically tether their psychology to unfitting brains"+No. The mind isn't in the magic; the magic is in the mind.+The magic is not self-hosting enough to allow it to design its own vessel from scratch.
"Give the embryos special magical protection via spells or potions"+Maybe theoretically possible to prevent their death this way, but they'd still be severely handicapped from having a brain that does not function at all as a brain.+I have a "my brain is bad at being a brain" tag, and I am nowhere near this level. This is "my brain is a genetic train-wreck which can't into neurology at all”.+Besides, no one has tried to do this, because in a world with 600 angels and uncountable mortals they are fertile with, no one was this dedicated to tiger-fucking.
"if I suddenly woke up as an angel in your world one day, I'd know what my next project would be"+ ...Would you mind if I nicknamed you Tiger-Fucker?