FANTASTIC! I LOVE pong, so I thoroughly enjoyed myself.
You get full points for sheer style- graphics are cohesive and the music matches perfectly. Bonus points for bonus track at the end, gives a reward for winning.
Main menu and settings music tracks doesn't loop- or they suddenly stop, unsure which.
The shaders/filters are perfection, and bonus points for adding a curve setting. I played mine at .02, with both filters on, and it enhanced the experience. I would imagine that increasing the curve too much would make the lower left deflections more difficult.
Graphical changes I would make is make the rougelite path larger on the screen- 'tis a little hard to see the icons.
I used only the freeze skill my entire run- and could get a sneaky two score on good timing. That gave me a sense of accomplishment, so definitely don't reduce the timer on that skill.
I would add a burst into pieces animation on the bosses, just to really seal in that "you Won! thing.
Gameplay: I was cruising once I figured out you weren't doing angle dependent on movement style of pong, that you were using where the ball hits paddle determines angle style. In fact, my paddle got so big and I got so cocky so when I faced Darkness I felt TRUE FEAR. Fantastic design.
Bug: I can't read the synergies at the bottom, so I could see what they were called partially, but had no idea what they did.
I would add a "Spacebar to leave" at the bottom of the shop. I kept forgetting, lol.
SecretGem titlecard needs to be centered- and while you're at that, I would shine up the gem itself a little. Add the ridges and some shines, maybe a star-shaped sparkle to add pizzazz.
Overall, full marks from me! When itch ALLOWS me to rate it, heh.