As a dissociative lover who tried diphenhydramine 500mg I can tell that these two drug groups are really nothing like each other. I know that DXM is commonly mentioned together with DPH, that's what caught my interest in the latter (DXM is possibly my #1 favorite drug, don't laugh but I prefer it over e.g. heroin) and I'm happy that I didn't dose the DPH higher but as you might know do dissociatives have a huge "therapeutic window" over different plateaus and already very low dosages (say 5mg MXE or 150mg DXM) come with a typical feeling of disassociation which the 500mg DPH lacked completely. Then even Salvia divinorum, being a kappa agonist, has more similarities to a dissociative but it is also its own drug group and not a disso.
Weed can potentiate dissos but I wouldn't classify it as one. It's very different too.