There's a lot of problems with industrial ag and monoculture, but I don't think the way that we store and process the grain is really a problem. There's a few different things that a lay person might call a "silo". The taller/skinnier ones are actually silos or grain elevators, which can be used for grain storage. If you see one out on a farm, though, it might be more likely that it's for storing silage. As far as i know, there's no reason you can't have many types of plants mixed into your silage, and having larger quantities in one space makes the fermentation work better.

The shorter/wider silo looking things are "grain bins" for getting grain to the right moisture content. They basically have a screen on the bottom that dry air is pushed through to dry grain and preserve it.

Obviously, we want as much small scale, local agriculture as possible, but part of having a climate resilient system will probably always involve silos.

I don't know much about agriculture, but I'm French, which means 1/ I love bread and 2/ we need storage for the winter months. Grain silos are cool, as far as I'm concerned.

Yay world building! Here's my two cents of what I watch people do and what I learned they used to do from where I live: Here's traditional grain silos from Northern Portugal. You can see from the crosses on top that grain (or 'pão', bread) is very important - sacred to the people. There's still so many religious festivals where the locally grown grain is central and the catholicism might have been added later, and this kind of underlines how desperately important grain is to people. The funny stone discs keep mice and other pests away (some feature that is essential for any grain silo, ancient or modern!). There's openings in the walls because you cannot seal the storage hermetically - it needs a certain air flow against mold. Some villages have one per family, but I think there were communal ones as well, just as there were (still are in some places) communal ovens where the grain was turned into bread (broa de milho). In ancient times these 'espigueiros' were used to dry millet, then maize got introduced from South America and that's what people grow still today.

I know very little about actually growing grain, haven't done so myself other than a little maize. But when you imagine a landscape which grain is grown makes a difference: there's summer grain (maize, wheat) and winter grain (rye). Some grains are more water hungry - maize is notoriously so, and the current situation here is that people pump a lot of water to grow it and that it's not great when you already fight with drought. I don't know how millet compares to that, nobody grows it anymore. Rye you can seed in late autumn and then leave it alone, no extra water needed. It can cope with temperatures under 0°C. Wheat I have never grown.

All grains give you their straw as byproduct, which has a million uses from animal bedding to basketry.

I imagine that a modern grain agriculture would (re)introduce grain to helpful companion plants (and other organisms). Companion plants protect from pests and keep the soil healthy. So instead of creating a dead landscape with our grain agriculture where we kill everything on the field that is not a grain, we would try to find plants or other organisms that can protect each other, maybe even for a harvest of different crops from the same field, like 3 sisters. Grain field size would be limited, instead of 1000s of hectares with only grain you would have rows of trees: fruit crops, wood crops, ... , in between the grain. This invites birds who will take care of many pests that would otherwise threaten your grain (although you will have to negotiate with the crows I guess, they will definitely want a share of your grain! Maybe you could call your local crow befriender and they'd make some kind of deal in your behalf?)

I do not know too much about grains either, but you can do intercropping with grains. Milpa for example often involves maize. Also barley and oat can easily be grown on the same field and have been for a long time. Also rotational cropping works very well.

At the same time perenial grains are a thing and they are amzing. Grasslands are extremly good at capturing carbon and transfering it into the soil. That can store tons of carbon. However that only works with perenial grasses and they are rarely used as food today, but it is an option.

Honestly the biggest way I would see it being used in a Solarpunk future would be the use of it as a community storage method.