This is an extremely small space. Where do you want a walkway? Where the pavers are? To the back of the house? Where would you fit railroad ties?

Planting it up will probably solve your erosion problem. What zone are you in? I'd be tempted to do a low-growing groundcover in the bare area near the flowering shrub, so you don't cover it. I like creeping phlox and tiarella. I'd probably put a very low sedum next to your walkway.

It is a very small and awkward area that leads us from our back porch to a walkway leading out of a fenced in yard.

We live just outside of Atlanta, Ga
The railroad ties would be added on to the existing ones to help with erosion.

I would extend the downspout so the elbow and drainpipe are at ground level, then enlarge the bottom step as a landing over and atop the system.

It might be possible the landing could extend to the gate as additional elevated boardwalk. That or you need more flagstone.

Add more rocks and boulders to the slope to prevent erosion and place your plants among them.

Thanks!

Wow! This is fantastic! Thank you so much!!!

Upkeep in a small sloped area is a pain in the rear; keep it simple, easy to maintain. Cover it all it one nice ground cover and let it densely fill. Liriope has deep, hardy roots that are excellent for soil erosion control. I have planted it in areas just like this. Here more info from NC State Garden Extension Program. LINK

It is a really nice plant when grown together in a dense area. It has a lot of texture.

Liriope is considered invasive in Georgia according to the link.