Difficult to get "lots of colour over an extended period" with just roses, even repeat flowering ones, so, yes, I would think you'd be better including other things.

How high maintenance do you want this bed to be? That will feed in to your overall approach.

A bed as large as this gives you space to go quite high, eg a small tree and a couple of rambler roses grown up some support - this gives a chance to extend the display by combining them with another climber to give flowers in a different colour or at a different time.

My approach (and this is a personal thing) would be to go for interesting foliage and let the flowers take care of themselves. And I would be looking for colour particularly in winter, when you really will be looking at this bed from the house rather than sitting out in the garden. So I'd be thinking of, say, one of the really nice white barked birch trees or a Prunus serrula with shiny red bark, a good Hamamelis to be covered in yellow flower in early spring (not sure whether this likes too sunny a position).

I'd have bulbs for early colour, while the ground is still relatively bare and not shaded by trees and shrubs. And I'd make sure I had a good berry display for autumn.

Once you've sorted out the miserable bits of the year, then it's a matter of filling in for the summer display., even to the extent if standing pots in any boring bits. The more of the ground you can cover, the less weeding is needed, although I know this is a matter of taste - a lot of people admire a garden of weed free earth dotted with brightly flowered bedding plants.