You have to do qbank and realize which ones are the important concepts to nail down even to the grittiest details
For example, some of the embryo (like you mentioned)/developmental milestones are probably not worth memorizing
the high yields for step 1 are path, pharm, micro, and physio. you should certainly study your embryo, but don't invest too much time there; put it in the aforementioned 4 subjects. on step 1, you might get some questions on a structure deriving from which aortic arch/pharyngeal arch/brachial arch, etc, and you should memorize those b/c they're easy points, even if there might only be 3-5 embryo questions on the whole exam. but definitely know the anatomy in FA.
as you go through uworld, you'll see how almost anything or any word in FA can be tested [hence you'll be advised to know it inside out]. certainly read and try to memorize all the embryo in first aid (i.e., you've been exposed to it once) b/c it can pop up on step 1, but don't invest the same time in that as you would for cardiac pathology (i.e., you might get more murmur audio questions on your step 1 than embryo questions total]. absolutely do not consult outside resources for embryo; just know what's in FA and UW. good luck!
Know FA cold bottomline, so many times ive seen questions ive missed on UW thinkings its one of those wtf answers, and think no way its in UW, i open up the big bad FA, and there the answer is, even if its in some small print in a graph, its there. also in UW lots of questoins i get right through straight recall, ie strawberry hemangioma grows and then regresses, sattar doesnt mention it but FA does, hence FA + UW + pathoma= Champ.
to do well on step 1, you should do your best to know first aid inside out, but choose wisely where you invest your time. return on investment is key. Out of 309 questions, you might get 150 path questions, 30 micro questions, 50 pharm questions, 30 anatomy questions, 5 embryo questions, etc.
if you want 230, then maybe you'd want to know 15-20 of those 30 anatomy questions. if you want a 250, then you'd want to aim for getting 23-27 of those anatomy questions right. and so on. to do the latter, you'd want to know first aid inside out, and that would require spending time on those little nitty gritty details that are less likely to show up on boards, but ones you still need to know if you want a high score.