I hear you.
It’s been interesting to trial different saddles and see how they behave on my horse. The latest ones were a bit too wide. Before we tried one that was a bit too narrow. Before that we tried some that were too straight and too curvy. Too long. Starting to feel like Goldilocks over here hunting for something just right.
My sheepskin lined pad seems a wee bit less prone to slipping based on how the saddles behave when I’m doing up the girth. I like the concept of sheepskin against the horse’s skin. Looks cute too with its fluffy roll up front. At any rate, since that’s the pad style I want to use on the regular that’s what I’m using in trials. Fitter is aware.
I checked out the Ecogold non slip pads. They do look very nice. I’m not having any trouble with the pad moving so there is that. Fitter seems to think flocking adjustment and a narrower tree will do a lot for my current issues.
One saddle I tried slid forward onto my horse’s shoulders, too narrow was the final verdict on it. Pretty disappointing as it was a very nice saddle and if it was made in a wider width I would have purchased one. I was venting to a friend about it and they were like oh just put a crupper on and it won’t slide forward. 
In my personal experience, correction pads like wither / seat risers destabilize the saddles. Even my shimmable half pad seems to destabilize saddles laterally. Might just be me and my horses but that’s my experience.
My retiree is using an old adjustable gullet Bates saddle I have with a wither riser pad on his quarterly walk hacks. I have to be super careful mounting up as it will slip sideways. More often his walk hacks are lead line pony rides for my young cousins. I hold the off stirrup when they mount to prevent slipping.
If I can ever get my main horse suitability equipped (without breaking the bank) I will move on to finding something that fits the retiree better as farrier would like to see retiree in a bit of light work and I’m not willing to work him more than his current occasional walks in an ill fitting saddle. Hopefully he’ll be less trouble to fit.
I’m certainly learning a lot about saddle fitting though! Guess that’s the silver lining.