I know sunflower seeds, flax seeds, chia, soy, and chickpeas are not considered grains. The protein content is there, but is it enough to sustain them until they are out of the brooder?
Not an expert, just looked into this as I'd like to improve my pasture/forage area to supply as much of my flock's feed as possible:
There are a lot of "false grains" like amaranth, teff, quinoa, etc that are other possible options. You can also use seeds from vegetables where if you are saving your own for replanting, you end up with tons of extras. Radishes, pumpkins/squash, etc. For chicks don't forget to make sure they have grit to process these whole foods. If you are feeding them strictly plant foods, make sure you get a complete set of protein (you want to cover at least all the essential amino acids for chickens to be sure they are getting the right kind of protein).
I imagine if you want them to free range as adults for most of their food what would be wise to do is to seed your range with plants that are good sources of nutrition and food for them (as opposed to an ocean of grass). Some of these sorts of plants could double as good cover from predatory birds (such as blackberry brambles, etc). Add in that food sources mature at different times of year, and it'd require some care to make sure you provide ample opportunities for foraging year round.
Additionally, making bug habitats might be wise too, so you can 'grow' bugs for them to hunt. I know there are bug hotels designed to attract beneficial insects, not sure what you'd do for just attracting feeders though. Maybe take pest infested/bad fire wood (far away from any wood structures) and make a pile to slowly decompose while providing an insect hunting ground? There's always ye olde compost pile to scratch in too.
Another consideration is how you will handle low forage food months (not sure where you are or how your winter is, but it may be unreasonable to expect them to find their food all year round). Whether this is saving up false-grains and seeds along with food scraps and leftover meat, raising bugs for chicken feed, growing fodder for greens, or something else.
Anyway, those are my thoughts of things to look at or consider.