Pretty house. I think I've seen you post it before.

First thing I would do is throw those rocks away. Or take them to the backyard. There's nothing easy or maintenance-free about rocks. And they clash with your bricks. If you feel you must create a bit of raised bed, use something similar to the bricks of your house.

What is your vision for the planting strip along the city sidewalk and the fore portion of the lot line? Unless you had a specific vision, I would not have done any work before completing the planning. Personally, I think your yard would have been better off with that area left to grass and concentrate bed expansion along the fenced area.

Given that the bed has already been expanded, I'm going to criticize the shape of the bed and pair-of-boobs-shaped lawn. It is just not a geometrically pretty combination! There are too many curves that don't quite match and they make the bed to skinny in some places.

When a bed contains a tree, the bed should be sized and placed as if it was a container capable of supporting the tree's life. Here, the tree is right on the edge of the bed, more how a weed would position itself in a container. Either put the tree in or near the center of the bed ... or, if the tree isn't moving, expand the bed so that the tree is in or near its center, never on or near the edge! If the bed is circular (like the tree is) it shapes and frames the trunk giving it added importance and finishment as a picture frame gives added importance and finishment to a picture. IMO, a circular bed around the tree would have made a great end to the bed, rather than continuing the bed to the city walk.

Now, if you're going to have the bed continue to the city walk, it can't be so skinny there that you cannot plant something meaningful in it. It should be at least 6' front-to-back bed depth. Deeper would be OK, too. Along the lot line, it should be at least 5' depth. Deeper would be fine there, too. Back to your vision of this area ... are you trying to add privacy with taller material? ... or just some interest, but not privacy?

Here are some goals I would aspire to ...

Just some interest, but not privacy. And we wanted less grass to mow.

Less grass to mow is probably not going to deliver less maintenance, but more. Those plants you'll put in instead of grass will require weeding, edging and possibly trimming, depending on what you plant.

If privacy not a goal (great!) then mostly low height plants. But the advice on the bed design still applies. Since you didn't react to what I've already said about that, I have no idea what you're thinking.

We are going to make the bed bigger around the tree and also next to the city path. Thanks for pointing those things out.

How many different varieties of plants/shrubs/grasses and in how many different colors should we target? Are there general rules of thumb, or is it just personal preference?

Even on this forum, you will get widely varying answers. There is the contingent who believe that form follows function and less is more (usually!) And there is the opposite contingent that believes more is more and that whim -- whatever you want to do -- is superior to rules that designers and architects employ. Each of these camps usually loathes the ideals of the other camp, even if they will not say much about it publicly. I will admit that, because plants are pretty, even when there is too much variety, the result can be tolerable if some of the basic rules are followed: don't block things that should be viewed (taller in back/shorter in front; don't mix incompatible plant types -- cactus and water lilies; keep things tidy/trimmed, etc. But even if too much variety is tolerable/acceptable, I ask why go there instead of making an overall stronger looking, organized artistic statement ...?

I think here, you might mark the corner of the property with a medium-lowish shrub 3' to 5' height in a dome or bullet shape. There could be something colorful near the driveway. There would be something colorful and exciting surrounding the tree (one thing only for strength). There are things like what I showed in the drawing near the house. The rest would be a single groundcover that extends to connect the things all the way to the front of property. Could you tweak it with additional interest? Sure. Could you clutter it with a multitude of other plants? Sure, but you are going downhill in appearance and adding to maintenance.

I think it would be easier to do something like this:

Whatever you plant will need maintenance so I don't think you will have less yard work. I actually have a bed on one side of my front garden. I didn't do it for less maintenance I did it for variety, so I am not trying to dissuade you. I just want you to have realistic aspirations. A bed like this needs what I call "grooming."

Will you be needing drought tolerant plants?