After reading through part II of pihkal and tihkal several times, it does seem that Dr. Shulgin would explore an exciting area for a bit, lay the groundwork for forthcoming scientists, and then move onto a new group of compounds. I'm sure he could have made every single combination of alkyl groups up to di-octyl-tryptamine, on every single n,n-, 4-ho, and 5-meo tryptamine, and dozens more 2C-T-X drugs, but after making 2C-T-21, it's as though he says "okay, this has been really exciting, but I have other things to discover".
I hope I get to see in my lifetime the pseudo-2C drugs (that is the 2C-X analogues with a 2,4,6 ring substitution pattern as oppose to the 2,4,5 ring substitution pattern). I wish there was more entries for those than just psi-DOM and psi-2C-T-4.
...it's interesting to consdier that there could even be all the NBOMe verisons of the 2,4,6 drugs...crazy ! endless amounts of psychedelic drugs.