As a newer cyclist, I'm going through some of the same issues as you. At times, I too tend to slide forward on the saddle, but I've attributed this to the following...

Saddle shape - I'm using a Specialized Avatar during the winter, the curve, or back to mid-saddle slope naturally pushes my hips forward during hard efforts. Changing hip angle is part of the saddle design, to avoid this, I'll need to change saddles. I also have a Specialized Toupe where I don't experience this problem, that's because this saddle is flat without any slope.

Saddle height - I've increased saddle height a few mm to improve spinning. Again, during hard efforts, I think this contributes to moving forward.

Saddle set-back - I need to have this checked, again. I've replaced saddles a number of times and even though the position was measured, it feels off.

Stem length - If the length is to long, you may not have enough flexibility to reach without sliding forward. I started with a short 80mm stem and recently switched to a 110mm after improving flexibility. You may need to try different lengths to find what fits best.

This is where everyone will chime in with bike fit, yes, a properly set-up bike will optimize your body position on the bike.