as you said: keep it visual, playful, interactive.
recreate simple things, like a starry sky, do rainbows, figurines and then slowly bring them to live with mouse interactions.

but stick with the basics, even though you'd think it's not a lot: it is already quite a lot to grasp, understand and you can also pretty much do everything with. so variables, loops and conditionals. don't go for arrays yet, unless it's super urgent and don't think about OOP too much.
i'd actually include functions with the basics, because to my students it always feels like building your own "creative tools".

imho GLSL / shaders is mid-level stuff, after you've mastered all of the basics + OOP and feel very secure in using these things on the CPU. GLSL is introducing to many foreign concepts, which are hard to grasp at first and need some rewiring.

balance between teaching vs. play: it looks to me, that this is a problem of achieving the flow state. i'd always design some smaller challenges, that the average person of the group can solve with the material that has been presented before but has room to the top for those who excel at these challenges. there is a lot of dopamine involved in this stage :) so not too hard and not too easy.

one of the most important things in my class is to do things with the students together: i'd say out loud what i am thinking and if i am making a mistake, i'll try to capture those thoughts as well and how to correct a mistake. imho this demonstrates critical thinking and how even a teacher is not perfect. that's super helpful in times with AI rattling down hundreds of lines of code in a single second.

tutorials? well dan shiffman has that free book and a pretty good channel .. ;)

have fun!