Thanks for the compliment on the pixel art.
There are many nice beginner tutorials on YouTube. I especially recommend AdamCYounis. Generally when making pixel art it is important to keep iterating on it until you get to a nice result. For color palette you can check out https://lospec.com/ . It's where I got mine from. Just make sure to credit the creator.
About the effects not showing: That's strange. But you said that your PC can't render Godot games in general. That might be worth looking into. But without further information I can't really say what the issue is.
The after image should display tho. Only some moves that are unlocked later use it, so you might not have gotten to those.
(also there are upgrades for the boost but there should probably be one earlier)
Thank you for playing :D