Cactus is a great candidate for sure, she has good abilities along with a higher than average skill activation chance. Lv 5 skill puts her at 46% activation rate, which is around 10% higher than your usual 6* girl.
While Scotchbroom's +15% act chance for the whole party sound OP and great, you're still playing with RNG at the end of the day. I do admit that it's fairly amusing to see it proc from time to time in ways such as pushing Crystal Room damage to over 3M, but it's still not reliable. It's best to stick with damage buffs or +weakness buffs for more consistent results.
Generally for crit parties, the girls involved often lack damage multipliers like Skill Damage and Boss Damage (And of course, weakness multiplier). So it's recommended to fill in that gap that once you have reach 80% Crit activation rate for that party, rather than over saturate the setup with more Crit Damage buffs.
Good example of such "buffer" (Yeah, I know that sounds wrong to call your girls that) are girls like NY Edelweiss, Lotus Bride, RG Elm, Purple Columbine. and many more.
Of course, it's a waifu game after all so picking girls you like is more recommended.
For instance, pushing RR's to maximum damage potential is actually not in a full critical setup. The gif below is RR doing 2.2m on 1 RB point, credits to some user on wiki.
Still can't post direct links lol...
gyazo.com/50db5015a3611bd2fcf33bcf5e2ca96a
They're pretty much comparable, but Sakura can probably make better use of the ampies at the moment simply for being a boss fight setup character. The difference between a fully ampied and none ampied girl at end game is whether or not she can eat a single hit and survive.
Don't need to worry too much about ampies as you would accumulate enough for everyone over time (probably).