I agree, soma definitely parallels weed much more closely, but I don't think it's a perfect match. Huxley imagines a drug a bit more insidious, without obviously negative side effects, and with somewhat unrealistic(imo) intended effects.

I'd say something with the intensity of weed (relatively low) along with the effects of MDMA. Essentially "MDMA lite"

Marijuana often seems to promote thinking "outside the box" which is probably not what the Brave New World people would want for their population

It is, you're right, and it's super weird what happens on the internet when you suggest weed isn't some gateway to enlightenment. I love cannabis, but it's a depressant that increases dopamine, it's not that complicated. Stoners on the internet sound exactly like alcoholics—they say it makes them more creative, helps them sleep, deal with anxiety too. We do such a shit job teaching about signs of psychological addiction.

The sedation is psychological - soma suppresses discomfort and boosts easy pleasure. It’s not introspective at all, which makes it much closer to MDMA than to cannabis.