I’m about grade 5/6 now, having taken up the piano again (I’d only ever passed grade 2) some years ago, after more decades than I care to mention here!
So I almost had to start again from scratch and my sight reading was virtually non existent. The tuner who came to sort out my new (old) piano said the way to go was just to get lots of music and just play it (or try to) without worrying about getting it right.
I bought several cheap 2nd hand books from abebooks or amazon. One graded series (Read and Play) was specifically for s/reading, starting with very easy, short pieces.
It took a while, but my S/R eventually improved dramatically. It’s IMO a question of developing instant short-cut connections between eyes/brain/fingers, which only comes with practice.
As for practising in general, after a while of working at it on my own, I found that I was doing it all wrong, just playing pieces all through over and over, making the same mistakes over and over. I belatedly learned that what I needed to do was find the optimum fingering, write it on in pencil, stick to it religiously, and then play any tricky bits repeatedly, but slowly.
Apparently 70 repeats ought to ram it into your muscle memory!
I’ve only ever learned 2 pieces entirely by heart, both with very regular rhythms and distinct sections - both were G5 exam pieces. I dare say it’s an age thing - I never had trouble as a child - but memorising anything at all ‘wandery’ is just beyond me now.