Secondary heart, ossmodula and biscopea should only be able to be put into children (or into someone with the "*insert primarch* gene seed" gene that I talk later that wouldintroduce a bit more work but be vastly better for customization and lore friendliness), in that order and making them either die or grow immediately into hulking children when the biscopea is installed(that way we are lore friendly about the whole astartes making progress), maybe even turn them into a whole other race (which would be some work now but I think down the line would help a lot for customization), then you have until they become adults to install the rest, and while some people said it takes years and are right, children in rimworld "grow up" faster for balance reasons, so I think that is already contemplated by the game...

As for requirements, all of them should require the previous one except for this ones:

Betchers: Rogal seed is missing it (so mainline and successors too)
Melanchromic: One successor of the raven guard (cant remember which one) does not have it... But the mainlines do.. so.. Up to you
Mucranoid: Again, raven guard.. Missing it (also betchers if I'm remembering correctly)
Sus-An: The fists were missing it but it was retconned to just be deficient, because it messes too much with astartes being walking tanks too much.. I think it was rogal that didnt have it at first and then they just made it deficient but then decided it was stupid to make the fortification astartes not be able to survive extreme damage.. But as everything with 40k lore, its up to the writer so... Up to you..

So only those should not be required by the next pieces of the gene-seed, and that can be incorporated into the main mod so you dont have to make a patch..


As for what the perfect solution would be:

A way to install the gene-seed of a specific primarch, and each seed is then upgraded in steps and following lore.. For example, first gene is "Rogal Dorn Gene Seed", and this first seed could carry the specifics of each one, for example, in rogals case, besides not allowing the betches and reducing the sus-an membrane efficiency (which I still believe should not happen as the retcon does make sense), it could give a bit more HP or even set a need for "pain glove" as these dudes are very into self hate... This way you could leave every organ untouched (even if they have changes depending on the primarch) and just push the primarch influence on a first gene that regulates all..

Next step would be secondary heart, ossmodula and biscopea (cause those are installed at the same time because of how they work), require the first seed and in this case, because you have the rogal dorn gene seed on, you cant get your betchers...


Again, if a git is available, id collaborate with it and use your templates to add different chapters and gene lines.. :D