ALL tree roots spread......that's just the nature of trees. Some types do generate a lot of surface roots compared to others but I doubt that is of much concern. The strip is too narrow to accommodate a tree of any decent size regardless of how its root system behaves. And unless you get and plant a pretty mature one at the outset (not advised!!), there is going to be an ongoing branch conflict with the walkway.

A groundcover makes much better sense.

We have a Japanese maple about 8' off the west side of our deck. It's 12' tall and does nothing to block the sun. That's about all that would fit in your space. You'd need a very tall and dense tree for shade.

Where are you? I’m in the deep south and a Natchez white crepe myrtle would work great in that spot- dappled shade in the summer but lots of light in the winter

Thanks for all the comments...I am in Seattle... Yes, groundcover or a row of shrubbery would be more suitable. I actually placed those blocks as I regularly run/drag a garden hose from the hose bib...didn't want to drag the wood chips ...so ran that strip of blocks...I will focus on a groundcover option.

You don't have room for a shrub or tree. You can see all of your existing shrubs are larger than your space. I'd look at groundcovers or subshrubs. A groundcover like moss phlox with a series of bulbs planted in it can look really great.

I'm in Portland. Like Seattle, hot is relative. Could you grow a strip of hakone grass there? The gold would be so pretty and it looks like three would fit once mature. Mine can handle a lot of sun. I have divided it many times and tried it under various conditions. On our 90+ degree days, the leaves will start to curl in lengthwise to let me know it needs a drink but it doesn't burn or die out.

Rocks or pavers. If you want a tree, set it somewhere over to the right.

Fill it with stones

Yeah, some stones as suggested...maybe along with something that grows well between the cracks.... any complementary greenry/groundcover that would do nicely with stones? (Seattle Climante)

Cotoneaster. Goes great with rocks.

But honestly, the Seattle area is gray enough as it is for a big chunk of the year. Why add to it with more hardscaping? Find a full sun ground cover you like and just use that.

My blue star creeper has spread nicely over the years. In her thread here, KW PNW showed her elfin thyme between flagstones and it's gorgeous. I think she's in the Vancouver area. You would want to use only a few flagstones to make room for the groundcover.

Like Celery's suggestion of stones. Perhaps Japanese-style paving?

The Japanese Paver stones are very welcoming feeling. I too was gonna say river rock but those @simplynatural stones are fantastic.

UPDATE 5/27/24: Okay, if anyone here is interested...I planted a few varieties that by their descriptions seems to be a solution for this small spot. From left to right: 1. Speedwell, "Snowmass " Veronica 2. California Fuchsia and finally 3. Red creeping Thyme....small for now, but I expect before I know it...it will fill in this space nicely 😀