IIRC, Roto Rooter makes some kind of enzyme treatment that comes in a gallon bottle and is supposed to help clear out fats/proteins and things like that. Typically, it is supposed to be used once a month to keep the entire water line system clear, but it can be used in a pinch as a “soft” drain cleanser. We had a stopped up sink a few weeks ago that I used it on because I didn’t have any of the “good stuff” on hand, let it sit overnight, and cleared the sink line right out and it’s been flowing well to this day. It should work if your odour/grime problem is organic (i.e. fats and grease, food, etc.) but won’t work if its scale/deposit/corrosion or something else inorganic.
Someone else had recommended an enzyme treatment, too — from Home Depot or a similar store. Before I posted this thread I thought this might be a worthwhile approach. I suspect the problem may actually be tied to the low level of use of the kitchen sink — meaning there may be some minor buildup of organic solids (coffee grinds, etc.) in the drain pump tank because the sink isn’t used frequently enough to activate the pump on a regular basis. Even just filling the sink and emptying a couple of times a week may have helped reduce the odor — even without the snaking.