Boi, how did you even have the patience to do breed 5GP pokemon, and how did you not even have a problem with hard mode, and the RNG with getting platinum and shiny cards for specific pokemon, I NEED TO KNOW YOUR SECRETS.
You can basically guarantee 5GP dudes by carefully examining egg groups and making clever use of power seeds. Some people find this sort of process intolerable but I personally think it's extremely relaxing.
We start with any breedable pokemon that doesn't evolve, or that evolves very easily (eg; Vulpix). I picked Absol because it's in the Field egg group, which is the largest, and also I like Absol. The grindy part of this is catching 32 of whatever you picked (I realized Absol was a poor choice about halfway through when I noticed it has a catch rate of 30, but whatever). Grab some Giga Balls and something with False Swipe and you can do this in 30~90 minutes depending on your target. Name all the males in the pattern A/B/C/D/E. etc, and the females in the pattern 0/1/2/3/4, etc. You need 16 of each, so up to "P" on the males and either "16" or "^" on the females, depending on how you notate.
Pair the males and females off, and breed one egg from each couple (use the Sask Incense to greatly speed this up). This will yield +1 GP offspring with about 96% probability. Name the offspring in the pattern (FatherMother); so you'd have an A0, a B1, C2, etc. You may need to breed one or two couples more than once to get a proper gender ratio. Now you can immediately pair off these offspring, again gaining +1 GP with about 96% probability. Repeat for two more generations and you'll wind up with two 4* pokemon who share no genealogy. You can breed these as much as you want to get 5* mons. The pathway in Shawmonk Calm is infinite, so you can just hold left and watch some anime or something while you're hatching.
To save a tremendous amount of trouble, we can now pass on our GP through chain breeding. Suppose you, like me, wanted a 5* Charizard, from the Monster and Dragon egg groups. You'd do this:
--Go catch a female Rhyhorn in the pit above Shawmonk town
--Give Absol a Power Seed (you can fish them up at the circus), toss the pair in the daycare, and save.
--Breed Absol with Rhyhorn until you see an egg with five stars (seems to be about a 10% chance from my testing). Don't forget to give Rhyhorn a Sask Incense.
--Hatch the 5GP Rhyhorn egg; we're aiming for a male one here. You can check the egg's gender before it hatches by naming it.
--Now you have a 5GP pokemon in the Monster group to Power Seed with a female Charmander, yielding a 5GP offspring.
--You can then use that Charizard to breed dragons, etc. etc.
You can stretch one pokemon into most egg groups in this way. If you're lucky or patient, you might hatch a pokemon with the little heart icon -- this is a permanent Power Seed effect. All male shinies have this innately.
The initial breeding setup takes about three hours total if you're efficient about it; after that it takes like 10~15 minutes per dude plus some Bulbapedia browsing. Some pokemon are harder to get females of (Eevee, any starter), so that can also add some time. I swear it takes me forty eggs any time I want to hatch a female of a 1/8 male species. It's also possible to skip a bit by inbreeding, but that makes the results more random and decreases your shiny odds.
Last note, Valkyreon is special: it changes egg groups when it evolves from Eevee (although take care; it might also change gender due to different gender ratios). That's how I passed on GP to my Omuran Machop.