Lola, believe me when I say I have wondered the same question as you, all because of a nearly invisible insect called Rose Midge. It is considered a significant threat to the rose industry in North America, not to mention individual gardens. The midge is a tiny fly that bores a hole just beneath each newly emerging rose bud where it lays its eggs. When the eggs hatch, the larva (which are smaller than the head of a pin) chew into the emerging rose bud, which kills or severely deforms it, never to bloom. Once fed, the larva drop to the ground where they wrap themselves into a tiny cocoon. Shortly thereafter they re-emerge, fly straight up into the same rose bush, mate, and repeat the cycle, over and over again, all season long - exponentially increasing their population. When roses are planted close together, they move from one bush to another like wild fire.
Imagine my previous garden (before moving) with 100 densely planted roses that all had healthy tall canes and arching branches loaded with lush foliage - and not a single rose bloom to be seen. I learned that the tiny midge was the culprit by using a microscope and reading up on this awful pest. It is virtually indestructible for reasons I won't go into (you can Google it) unless you are willing to drench the soil with imidacloprid, only to see all the critters that live just below the mulch, like helpful earthworms, come wriggling out and twist themselves up as they die. Of course, the rose plants take up the insecticide, so you know that the residue inside any flowers that may appear six weeks later could harm helpful pollinators like bees and butterflies. And this treatment has to be done three times during the growing season, season after season, just to keep the midge from repeating its spectacularly successful habit of multiplying.
It was more than I could take, both aesthetically and ethically. It was absolutely heartbreaking for me, to the point of real depression, since I had spent years designing, expanding, and planting the rose gardens - it had become a way of life that came to an abrupt halt. The perennials that surrounded the roses continued to perform and bloom just fine, but it just wasn't the same without the rose blooms. One of the reasons I love roses is that they bloom from May through October, whereas many perennials only bloom for a portion of that time.
As fate would have it, I moved several blocks away to a new home seven years ago, where I designed and planted the gardens where I live now. The buyer of my former home encouraged me to take as many plants as I could when we moved, because he was going to dig it all up and replace it with grass. However, this was just as winter was setting in (a few weeks before a major snow storm), so while I was still able to dig up 100 roses and 100 perennials and shrubs, I had to overwinter them in whatever pots I could find and purchase from nurseries that had mostly closed for the season. Digging up a few hundred plants and haphazardly repotting them in freezing temperatures while also packing up my house while my husband had a torn rotator cuff was almost more than I could deal with. But I managed - and I got a second moving van just for all the plants.
Disrupting the soil around the roses when digging them up seemed to kill off most of the cocooned midges that were hibernating in the soil. As (mis)fortune would have it, that winter was the coldest seen by the Hudson Valley in over a century. As a result, I lost half of the roses I potted up and placed in the cold greenhouse, but it may have also helped to kill off the midge.
When spring finally came I replanted the surviving roses, but refused to buy any new ones until I could determine whether it was pointless to do so, not knowing whether the midge would reappear. Also, I vowed to plant my roses further apart, so as to help limit the spread of any midge that might survive. As it turned out, the midge did reappear, and continues to do so, but it has been limited to a few roses. So I purchased large sheets of brown plastic tarp and cut it up into circles about a meter in diameter. Wherever the midge appears, I place one of these circular tarps beneath that rose to catch the midge larva when they fell from the rose bud, to prevent them from reaching the soil for the cocoon stage. This has been very effective in breaking the life cycle, and I've learned to live with the occasional tarps here and there. They aren't very pretty, but at least they match the color of the garden mulch I use. To my surprise, I haven't had a single rose midge infestation this year, and it has been a wonderful respite. For the first time in 7 years, I added a few roses to the garden this season.
But let's return to your original question of what I would choose to grow, if not roses. When I planned the gardens at the home where we live now, I made a very conscious choice to diversify the plants far more than I did at my previous home, because of the midge. This means that I spent a lot of time reading up on perennials and bulbs. I already planted lots of perennials at my former home, but when we moved I decided to delve deeper into my knowledge of a much larger variety of perennials than found in most local nurseries. I was particularly interested in creating a "continuously blooming" garden without having to invest time and money into planting annuals every season.
I found a few books to be very useful resources, which you might want to look into. They are:
"Continuous Bloom" by Pam Duthie, subtitled "A Month-by-Month Guide to Nonstop Color in the Perennial Garden" - published in 2000. (Amazon carries it:
https://www.amazon.com/dp/1883052238/) As you might expect, it's written according to a northern hemisphere calendar, citing North American perennials. But if you can convert the calendar to the southern hemisphere and find the same or similar cultivars Down Under, then you might find this book useful. The same goes for the next book:
"Gardening with Perennials Month by Month" by Joseph Hudak, second edition printed 2004. At Amazon:
https://www.amazon.com/dp/0881926345/)
"The Well-Tended Perennial Garden" by Tracy DiSabto-Aust, now in its third edition printed in 2017. This is a magnificent and invaluable book - it was a real game-changer for me. By that I mean the knowledge it imparts about each perennial's behavior and culture. Unlike the above two books, it's not about month-to-month blooms, as much as it is an encyclopedic review of individual plants in very practical terms. It includes a very valuable section on pruning and pinching, which was an eye-opener for me. Highly touted by the New York Botanical Garden. At Amazon: (
https://www.amazon.com/dp/1604697075/)
So rather than suggest individual plants for you, I'll leave you with the above information, hoping that it might help you deal with your frustrations, and envision something new and (almost) as satisfying as roses.
..... there's something beyond the reasonable of tolerating large scale destruction by wildlife on a rural property. Certain ammounts of wildlife can coexist reasonably well. From the numbers you stated you seem to have an outgrown population that will have to be culled. Your incoming neighbor with market garden targets to meet, will have far less consideration about the possums than you can imagine. He will have to take care of them and dispose them as allowed on your island. As a result you'll be able to continue gardening as before and perhaps thank that you now have a neighbor that does the dirty task... 
Other much less dangerous critters have passed my fence (that has wire mesh):skunks and they can destroy our laying hen coup in just one night. Again the dogs make sure that they don't get inside. 
