Your drawing is lovely!
You have a lot more room between the bottom and middle tiers than you do between the middle and top (there is a 2” ledge space between the 12” square and the 8” square; but only a one inch ledge space between the 8 and the 6 so you have room for a larger flower “posey” there than you do between the middle/top tiers.
You can wire a few flowers together but if you bundle a lot of flowers together, it might be hard to adhere them to the cake. I like to use a small, narrow straw (we know them as “coffee stir sticks” some are tiny round straws while others are flatter) to insert the wire of a flower into, and then put the straw into the cake (the straws are usually long so you have to cut them to fit). The straw protects the cake from whatever might be on the wire and also gives a little more “strength” to the flowers wired together.
So I would suggest grouping the flowers into threes and then placing them as in your diagram - you’ll be creating the mass of flowers by putting them close together.
Good luck! It sounds beautiful!