Where are you based out of?

California

You need plants that will help stabalize the soil. Go to the garden center where you live and find out what can do that. It looks like avery steep slope and the spacing is IMO very bad . I assume some HOA has decidied the plant arrangement with no idea of how plants grow .

I assume the land with the red arrow is yours. Is the land with the green arrow also yours?

And where are you planning to plant?

I woild see if you can tier this more. One more tier in the middle would help make thos a much more useable space and control the slope better. In our tiered garden we have boxwoods around the perimeter of the area right by the blocks and then behind that we have pachysandra and something. On one level
It’s ferns and a tree, on another level it’s rhododendrons and a mother tree. You will need to be aware of watering issues, what is suitable for your area and so on. But for me this slope, as is, would be too much and I would tier it more

Is the area you are planting is between the retaining wall and fence? The requirements of 5 feet from fence and 10 feet apart seems way overkill and something I would probably disregard (esp the 10 ft requirement).

Research “best plants for erosion control in California” to find species that will grow root systems that stabilize your hillside and prevent it from washing out. From a quick search it looks like California buckwheat and California coffeeberry are highly recommended shrubs for this purpose, and you could plant deergrass inbetween the shrubs/ closer to the retaining wall for aesthetics and to further help with erosion control. You may want to cover the hillside with some temporary jute matting and shredded mulch while your plants are getting established.

Not sure if the concrete blocks are the finished product of your retaining wall or not, but I would consider applying a stucco finish to it.

The original poster wrote "It has to be 5 feet from the fence and 10 feet apart."

Are you looking for trees?

Would you please clarify if this is an HOA requirement or fire safety requirement (Zone 0)?

Also, what is the nearest big city or Sunset planting zone that will tell us more about your climate?

https://www.sunsetwesterngardencollection.com/climate-zones/

There will be a lot of systolic pressure on that retaining wall. Are you placing drainage lines and weep holes behind it?

I like @WestCoast Hopeful’s suggestion of another wall/tiers. I did the same in my sloped yard.

Consider studding some large rocks in the slope to add stability and create planting niches.

Are you adding drip lines? Even drought resistant plants need water until established or during dry times.

I think you need to seek the advice of a local landscape contractor or designer in your area. Too many unknown variables to consider.

If you want something big, Toyon can be planted 10 feet apart and fill in nicely. Will take to pruning or shaping into a hedge or tree.

I would use jute mesh like you see along roadcuts for erosion control. Lay it first before planting anything.

Include creeping Rosemary to grow over the wall.