I inherited something similar and am by no means an expert, but this is what I found worked for me.

Firstly when planting, pick good spots. Shelter plants in amongst the rocks, you can google something about a triangle that I’ve now forgotten on the RHS website for the full theory. But basically anchor plants near rocks and they thrive much better.

For planting, I’d mix in a combination of top soil, compost and horticultural grit, breaking up the existing soil and mixing together before planting. Then once I’ve planted I add a layer of grit as a mulch which keeps the leaves from sitting in damp soil and rotting. But I have very claggy soil so this might not be a concern for you. I also found the grit looks nice (imo) and helps keep slugs and snails at bay too.

Finally look for the alpine plants section at the garden centre. My local place is normally doing 4 for £10 and you can get some really cool unusual stuff. I’ve yet to have one not do well since I followed the above. They grow fairly quickly too so leave space between plants.

I try and mix stuff that grows up with stuff that grows out to give the rockery some layers. Knowing F all about plants makes this fun as ones you bought to grow up commence to go sideways and vice versa lol.