Try

Deep Step Up

And

Weighted one leg hip thrusts

And

Bulgarian Split Squat

Whichever you can make hard enough that you can get to failure in 3 to 30 reps. Do three sets, stop a rep or two away from failure.

Then, over time, make it harder in one of the following ways each week:

-More reps

-more sets

-the same reps in fewer sets

-less rest time

-more weight

Hopefully this helps your glutes gain some muscle.

But just know, the only way to put fat on your ass is to gain weight.

Side note: you don't have to feel it for it to work. If you're doing a glute exercise and going close to failure, you're growing your glutes.

Pistol squats can be great if you've got the flexibility

I haven't done it but maybe some kind of hip thrust

Kettlebells could work well for you. Front squats, Bulgarian split squats, somersault squats for quad heavy movements. Swings, deadlifts and good mornings for posterior chain. Could implement some lunges as well. Some of those movements can even be done with your dumbbells, but to me nothing beats the bell for most of those.

As a cardio nut, I'd suggest biking, running, or copious walking. I know it sounds lame, but the simple act of walking can go a long way for that region.

dont we all

When you work out, do you actually feel like you engage your glutes? That has been a huge problem for a lot of my life and I believe if you have a weak posterior chain you can kinda lose the mind-butt cheek connection to get the muscles firing because then the rest of your body over compensates and kinda takes over the function of the glutes.

That being said I believe that good exercises for REALLY isolating the glutes are bridges. Make sure your abs are engaged and your tail is slightly tucked and squeeze those babies so that it feels like you're being propelled by the cheeks. You can hold weight on your pelvis to add resistance. If you feel strong enough you can do one-legged bridges as well. I think as with all exercise, though, you want to make sure you have the right form down and you're actively engaging the muscles you're trying to engage. If you're feeling the bridge in your quads/lower back, you're not doing it right.

Also I think resistance band exercises are pretty good, like doing the side to sides can help the gluteus medius I believe. I think donkey kicks are supposed to be good too. I'd also look at hip flexor exercises too as sitting for long periods shortens those muscles and I think that can also affect your glutes/posterior chain.

I hope this helps and sorry if I over explained, I just like to be as thorough as I can to make sense of things

Climbing. I started going to a climbing gym sometimes and I can feel it in my butt for days.

Proper single leg squats and step ups. You don't even need weights to start. Think split squats, shrimp squats, and eventually pistol squats once you have the flexibility. In the step ups in that program you linked, the non working leg is clearly pushing off of the ground and helping in the example video. That's the wrong way to do it. They should be done like this.

Don't sleep on barbell squats. If you do them right they will work the glutes.

Also some running.