Renovate entrance platform

Hello everyone

I could use a little support.

On the platform in front of the front door but also on a short path, slate slabs were laid at the neighbor’s place. Some of them are now loose and I was asked if I could reattach them.

I have attached some pictures of the situation.

I found silicone on the vertical slabs. This is, among other things, the large lump at the bottom left in the second picture.

With the loose step slab, you can see that the substrate is very uneven and the irregularities were full of sand.

My idea now was to close the hollow spots on the walls with mortar and then reattach the slabs with tile adhesive for natural stone.

At the spot where the step slab is, I would remove some of the current material and then also apply tile adhesive here. Although at the front left corner I would level the missing spot with mortar. Or can this also be done with tile adhesive?

Then I would tape off the slabs and then seal the joints with grout.

Do you think this would work or is there a better solution?

Thanks in advance to everyone.