Not sure if you got your answers or not, but I have been working on Pikarom since my son (Jr league) and daughter (Sr league) play competitively. The question really is how much will they have to deal with some of the threats Masters have to deal with. For example, most post-rotation Pikarom lists play a copy of Oranguru to counter the shedinja matchup. The problem is that neither of my kids have ever had to play against shedinja. They play against Reshizard and mirror Pikarom a ton. Below I will cross out the ones I think you should remove and add the ones I think below that.
Pikachu/Zekrom (2)
Jirachi (3) TEU99 (Remove - The Dedenne version seems to work better as the only means to search Jirach is Pokémon Fan Club, which isn't good).
Zapdos (2) TEU40
Tapu Koko GX (1) SM33 (Remove - Rotates out)
Tapu Koko (1) TEU51
Zeraora GX (1) LOT86 (Add 1 copy - Post rotation this card is amazing for free retreat, you can't afford to prize it. Also most decks play 2 right now anyway).
Tapu Lele GX (1) GRI50 - This is out I believe (Remove - Rotates out)
Absol (1) TEU88 (Remove - Not searchable and not as necessary post rotation).
Marshadow (1) SLG45 (Remove - Not searchable and better just to play higher stadium count)
Xurkitree (1) UPR160 (Remove - Not searchable and difficult to find the turn you need. No nest ball is a killer).
Lille (4)
Guzma (3) (Remove - Rotates Out)
Cynthia (2)
Volkner (2) (Add 2 - this card is amazing post rotation. The only reason not to play 4 is if you are playing Lisia and going that route, which would mean adding 4 copies of order pad and hoping to hit heads).
Zcerola (1) (Remove - rotates)
Rescue Stretcher (1) (Remove - rotates out)
Thunder Mountain (1)
Aether Paradise (1) (Remove - rotates out)
Choice Band (2) (Remove - rotates out)
Escape Board (2) (Remove - not necessary without Jirachi)
Electropower (4)
Ultra Ball (4) (Remove - rotates out)
Weakness Policy (1) (Remove - not as good. Consider perhaps playing a copy of Weakguard Energy instead if you want to go that route).
Nest Ball (3) (Remove - rotates out)
Switch (2)
Field Blower (1) (Remove - rotates out)
Lightning Energy (12)
Now let's talk about inclusions. I personally think 2 Raichu & Alolan Raichu GX is a must. It vastly improves your Reshizard and Mew & Mewtwo GX matchups, which will likely be 2 huge decks post rotation. With that card you can take a 1hko on either with 5 energy and an epower. Plus you get your RaiRai out of the way. I think you probably want to go with Lysandre Labs (2) and Viridian Forest as your other stadiums. The Lysandre labs beats shedinja and Reshizard as they play Choice Helmet to avoid the 1hko. Also, you want a high stadium count to counter Power Plant as every Green's build will be heavy on those. PowerPlant is devastating to Pikarom. I would add 2 Dedenne GX for speed. Also, you probably want to go 4 electromagnetic radar to discard energy for Tapu Koko PR and search. 2 cherish ball probably, and consider a mix of energy switches and tag switches to get those huge hits. I really think Mew & Mewtwo Gx should also be in this deck (1) as counter against fighting decks. Also, almost every deck post rotation should have 4 custom catchers and probably 2 reset stamps.
On the Supporter side I would play 4 Cynthia and 4 Volker, and possibly add a Lisia or two to get your prism stars. You could also play pokemon fan clubs and go that route instead, but I wouldn't play more than 2. The reason I say Cynthia instead of Lillie is that it is better post rotation. You will find a lot more hands with cards you can't get rid of (i.e. custom catcher) and you also don't want to discard. The value of Lillie post rotation isn't as good imo.
Hope that helps get you started.