I don’t understand the idea that any of his criticisms were oriented at neoclassical economics, or could’ve possibly taken it into account.

Many things happened after Marx's death and his critique still applies to. There may well be reactionary theories formulated in the future that my current politics would take account of anyway. Neoclassical economics is a continuation of bourgeois political economy that Marx wrote against. Not to mention that Marxists who have continued Marx's project after his death, have very much written against modern economists.

a neighborhood of people that has like a local nonprofit grocery store that is managed by the people who live there specifically so that people can have food and for no other reason. but maybe like a handful of people notice some problems with the way the grocery store is being run, but are having trouble actually getting people to listen to them so they decide to just show everyone what they mean by starting their own grocery store in the neighborhood too under the same exact community managed model

That sort of thing you describe is a common conception of what life ought to look like by a lot of anarchists, which is opposed by communists precisely because it preserves exchange, implies a division between town and country, and implies the preservation of many things which communism abolishes. It's also worth noting that when we talk about communism as a mode of production we are talking about society as a whole; for instance, a kid deciding to start a lemonade stand in a communist society wouldn't recreate class society as the kid is doing exchange.

A lot of communists stray away from "positive" concepts of communist society because it's much easier to derive what communism doesn't have than what it does have. We can, of course, look at humanity before class society, but a lot of things have changed since then, and it is unlikely that the abolition of class would lead to the primitive pre-class societies that used to exist. I'm disclaiming that not as a cop-out but because I think it would be facetious if I tried to give you an outline of what communist society would look like when really I don't think anyone can know for sure. But, most certainly I can say that what you outline does not sound like something which would exist on a large scale in a communist society. Most communists believe that central planning is a necessary part of a communist mode of production, myself included. Deciding that you don't like a "grocery store" and deciding to start your own sounds rather like capitalism, and suggests an individualistic economy rather than one where society as a whole collaborates. In a communist mode of production there is no economic distinction between individuals, between "grocery stores" as you call them, or between the individual and society. Like I said above, one person deciding to start a "grocery store" wouldn't cause the rebirth of class, but if that's happening on a large scale that doesn't sound like you've achieved a communist mode of production.

“well in capitalism ‘stores’ are places where people spend money so there’s literally no way anything remotely resembling this could happen in communism, not even if the food was free”

Things being "free" doesn't necessarily make it not a store or not capitalist, but a lack of exchange (among other things) does suggest communism, and I don't think the concept of a "store" makes sense without exchange. The abolition of property abolishes exchange. For instance, a food bank is not communist despite being a site where items are distributed for free; its existence relies on the alienation of the means of subsistence from a group of people, ie the existence of property. (that is also ignoring the fact that most food banks rely on a voucher system, which again is exchange, but if we were to pretend that food banks just give away food to anyone who comes and asks)