SparkSL is Spark AR's version of GLSL for writing shaders. Half of you probably stopped reading after that last sentence. Long story short, it gives you a ton of control over how something can look. In true Spark AR fashion, their online documentation has about as much substance as season 2 of Tiger King. Fortunately a creator in the community, Adam Ferriss, has a ton of great examples and I was able to figure out how to convert a shader from Shadertoy to SparkSL. I have a written version here (comparable to Tiger King season 2) and a video version here (comparable to Tiger King season 1).
While I was working on creating my SparkSL tutorial, the shader compilation step was basically immediate. Then I opened up OBS to record the video tutorial and things grinded to a halt and the shader code would take minutes to compile. Even restarting my computer did not solve this. Both OBS and Spark were using my laptop's integrated GPU, so I tried switching Spark over to my discrete GPU and the compilation process went back to immediate. I don't think many people are recording SparkSL tutorials, but if you run into this issue, you can try this solution (if you have a discrete GPU that is).
If you are wanting to grow your Snapchat audience, my fellow creator Phil Walton wrote "The Ultimate Guide to Snapchat" which goes over how Snapchat works and how to leverage all its different features. It is concise, to the point, and on my bookshelf.
AR glasses are not going to replace smartphones. I can see them being an accessory like smartwatches currently are, but I don't think they'll be replacing phones.
Spark AR and Cyberpunk 2077 hosted a filter competition a few months ago. I don't know how many filters were submitted, but I'm guessing Cyberpunk 2077 got a whole lot of filters and promotion for pennies on the dollar (if that). Spark is back with another competition for the holidays, but this time we aren't all being enlisted to market someone's video game, it is just a friendly competition.