You might like to put in some plants that will grow big (tallest ones as the back) then fill in between them with quick-growing annuals, that will be overwhelmed by the large ones as they get bigger
If you main plants will grow to (say) 2ft wide and 2ft high, put them in at this distance. You'd use a row of about 6 at the back
And about 3 rows
But use smaller plants as you get nearer the front
In my front garden, I have a summer bed of similar size where I put tomato plants along the back wall, followed by large decorative plants, followed by border plants, followed by small. It uses about 4 large decs, ten border, 30 small, and an infinite number of Alyssum. It is intended to have a lot of bright showy flowers for the summer
No reason you should like my choice, but the plants reduce from about a metre at the back to 100mm at the front of the bed, and the number of small, plants is greater than the number of large. It also holds over 100 tulips, which are planted deeper.
you will probably be able to reach to the back with a hoe. If you grow annuals from seed, the cost is trivial
