Re-vamping mortar in old stone patio

Long time lurker here. I'm a first-time homeowner with a perplexing patio. The house was built in 1940 and it looks like the patio has been around a while too. It's sturdy but unsightly so I am looking for solutions.

The patio is a bunch of large flat stones affixed to a concrete slab. Some stones look like flagstone, others look like they were just found around the property. The stones are STURDY on this slab so that is not a problem. What's driving me nuts is the unevenness of the stones. While some places are just fine and flat, there are a few spots that will inevitably trip you.

Last summer I powerwashed all the crumbling mortar between the stones and just laid sand to fill the space. This looked okay for the season but I eventually had weed growth and it didn't help the tripping situation. Here's what it looks like today (don't mind the snow...)



This spring I'd like to tackle the filler between the stones again. What's the best option? Concrete/Sand mixture? Joint stabilizing sand? Self-leveling compound? It would be great if the solution could help even out the surface. I'm in Rochester NY, so we get cold icy winters and hot humid summers.

Thanks in advance for your help!
M