Does it matter how much (dry) yeast you use in a bread/roll recipe?
I had already poured the whole packet of yeast before deciding on halving my cinnamon roll recipe (they turned out great) but if we imagine the other way around (half the yeast for the regular amount of flour) would it change anything? Especially if I'm not pressed for time so that it can rise as needed?
Yes, the amount of yeast matters (as does how old/effective the yeast is). How much it matters depends, but it definitely is a key component of bread. Bread is more of an art form than a science, and it can be somewhat forgiving of ingredients being out of balance with each other. But at its most basic, bread is air pockets (created as a by-product of the yeast consuming sugar or similar) suspended in a structure of elastic gluten (produced when you mix flour and water and knead it). The foundation of any bread is the balance between the air pockets and the structure that holds them in place.
If you add more yeast than the recipe calls for, the amount of sugar or other sweetener will still limit how much air the yeast can produce, but it might make the end product less sweet than it's supposed to be (since more yeast = more sugar consumption). If you add less yeast than the recipe calls for, in addition to probably ending up with bread that doesn't rise enough (and is therefore denser than it's supposed to be), you might end up with excess sugar and a sweeter end product than was intended.
Neither of those things is necessarily going to ruin a recipe, and there's a lot of personal taste that influences bread. But if you really have too much yeast, it is possible for bread to over-rise (which would stretch the gluten beyond its breaking point, and leave it unable to hold the air pockets suspended in the dough). If you really have too little yeast, or if your yeast isn't very active anymore, you might end up with bread that really doesn't rise at all. In that case, you'd probably end up with extra sugar in your end product, so it would also be a little sweeter than it's supposed to be. I think it would take a relatively large difference in the amount of sugar that was supposed to be left and what actually was before you'd taste much of a difference, which is why bread can be fairly forgiving of such an imbalance.
Ultimately, there's a limit to how much one little yeast cell can produce, so if you don't have enough of them (or they are particularly lackluster from age), you can wait forever and the bread still won't rise enough. The other piece of the equation is when you bake it, because bread rises in the oven until it gets cooked enough that the yeast dies. So if the yeast is rising very slowly, the bread will rise very slowly while it's baking, and at some point there is no more rising that will happen because the yeast is dead.
Flour, water, sugar to feed the yeast, and yeast are the only essential components of bread (other ingredients contribute to the taste or texture, but aren't strictly necessary), so mucking around with any of those essentials too much can throw the whole thing out of balance and result in really non-tasty bread. Mucking around with them a little is one reason why varieties of bread exist, and people frequently do alter that basic balance of flour/water to yeast/sugar so that they can have bread to their liking.