Don't know about ketamine or about sourcing controlled substances from the web or wherever (can speak to sourcing in clinic or hospital which I'm pretty sure we can rule out for BM) but have two quick thoughts:
1) BM is on record as decrying the alleged drug use of various "methheads" he's employed, and I'd guess that he might have had access to street-level suppliers had he wanted
2) The "drunk eyes" and allusions to SM "taking the edge off" (paraphrasing) with a drink or several strikes me as yet another instance of BM looking to take over the narrative by replacing a fact (SM apparently snapping a sunbathing selfie for her lover) with the SM of his imagination (self-medicating floozy resorting to drugs/alcohol while flouting his needs) -- a pattern we seem to have seen over and over in this case
3) If BM was on the darkweb he will have hacked around and left a trail -- just don't see him having the patience or focus to build a Tor setup properly and vapour-trail a Bitcoin and then ketamine or whatever purchase -- this is not a simple mechanical thingie, even with a Tor-loaded browser like Brave, etc.
4) We have only seen the bare frame of the State's case -- key witness and other testimony is likely to nuance that frame in a much more thorough and convincing way, but the basic scheme of preparation, execution and cover-up is already pretty striking, IMO
5) Few criminals, even professionals, are jacks or jills of all trades, IMO. BM, with his hunting experience and landscaper training and equipment, may have managed a more thorough or efficient disposal than most, but we've already seen evidence of slip-ups in terms of the texts, attempts to "silence" truck data and the trash run, and there will be more to come. I have no idea what a jury is likely to think, but even if some of this evidence is thin, its combined weight is significant. IMO, MOO, etc.