RCS look great on black...Flourite Sand is what I had my MA cherries on. Since you already had it, you know it's hard to vac...a good chance to practice your "swirl & vac" technique, if you've lost the hang & detritus studying looking for lighter than F. sand shrimplets. 3 gallons seems like maybe enough for your 29g, I'm trying to picture volume vs area...but when I bought mine, it was an opened bag & a 10g & cheap! 2 inches may be enough for most crypts...maybe 2.5.
Well, I have to say, just when you convinced me to maybe try oil dri, I'm disappointed it's "endangered" at Wally cheap prices (at least where you are?). I may buy 2 bags if mine still has it...next trip.