Yeah, the RDA on some things is probably not perfect, yet. And it varies by where you live. Near the equator, D supplementation isn’t a thing, so much. I live above 60 degrees latitude, so obviously I’ll have to take D supplements on top of Soylent.
I’ll also supplement with folic acid and fish oil, both to help with my arthritis–something the D should also do. (The folic acid is to keep my arthritis medicine from making me too sick to my stomach to function. And there was this really compelling study about fish oil helping arthritis patients, but only in fairly high doses–6 grams or so a day.)
Venturing further beyond things the RDA covers, I’m not ruling out trying turmeric and ginger supplements, once neither is part of my diet, because both have been shown to help with inflammation-related diseases and pain. The trick is finding a USP-verified source of either. Maybe they’ll taste OK just dumped into the Soylent as spices.