Since seeds are packed with nutrients, could it be a good idea to break them (so they don't sprout) and use them as a fertilizer?
Yes, they contain nutrients and as organic matter you could make a fertilizer out of seeds.
The seeds I find in this list even have passable NPK profiles.
I'm not going to argue their quality as a fertilizer, the problems that would probably occur with very starchy seeds or the work that goes into crushing a lot of seeds so they don't sprout (they can be pretty durable). The economics are pretty prohibitive in this case: At a quick glance the vegetable seeds with the cheapest price per gram for me would be beets. But for 500g of beet seeds I could get about 25kg of organic fertilizer or about 60kg of mineral fertilizer. I could also get about 15kg of buck weed seeds (or other plants popular as green manure) for that, but I would probably get a better fertilizer out of them by just using them as green manure.
So can get fertilizer with a known (and probably better) nutritional profile and bioavailability for a fraction of the cost and have a lot less work.
In conclusion: if you have a lot of time to waste, money to burn and like uncertain outcomes, using crushed seeds as a fertilizer could be a good idea.