Deep, dark secret: I too enjoy working in flower gardens!!
I’ve recently retired after 30 years in LE and military and, frankly, I’m ready to let the young guys (and gals) work weekends, kick doors down, root through dumpsters, do reports, consume caffeine in excess, and all that. I want to relax. (Shooting an AR-15 obviously also falls in the relax category.)
I’m no expert on flowers, plus what works here in Virginia may not work in your area. However, I can make a few observations.
The first deals with your back problem – I suspect you’re going to have factor this into how you do things. Working with flowers can really be hard on your back.
For example, I like annual flowers because of their vivid colors, but they have to be replanted every year. To avoid this, you might be better off with perennials, which last several seasons. Also, consider the types of tools that you’ll be using. A wheelbarrow with wheels on either side might be better for you than one with the single wheel in front. A small stool to sit on might help.
A local nursery can help you with what works in your area. Typically, they only sell plants that do well in their area.
Your lack of sun is an obvious issue. Many flowers won’t do well without a lot of sun. In particular, you may have problems with roses because of this. However, there are some flowers that do ok in partial shade and a few that like deep shade. Check the little tags that come with the flowers for this.
Impatients work great in shade in this area and are one of my favorites; however, they are annuals. Also, like other annuals, they’re going to be more expensive than perennials since you have to buy bedding plants every year.
Also check the tags for the size of the adult plant; sometimes cute little plants grow into great big monsters. (My community has many houses with bushes and trees growing into the house, driveway, each other, and everything else.) Obviously you don’t want to plant tall plants in front of short plants.
You didn’t mention drainage but don’t forget it, it’s important. Also, think about any animals you’ve got in the area. I’ve got deer, which have totally destroyed my roses. I’ve replaced the roses with salvias and marigolds, which are deer resistant and are working well (though I think the marigolds could use a bit more sun than they’re getting.)
You’re apparently already aware of the importance of initial soil preparation; do this right - don’t get in a rush to plant.
Don’t be afraid to experiment and change if things don’t work out too well. It can take several years to figure out what you like and what works well. Also look around and see what your neighbors are doing.
WARNING: If you want to put a lot of money and work into a flower bed and then have it totally fail, then just don’t water it for a while. Been there, done that!
Wish you all the luck in the world – don’t get in a rush, don’t hurt yourself. Remember, the real pleasure is in the travel to where you’re going, not in the arrival.