OKAY! I figured it out. I think. I went diggin' around in the code and threw in a million Debug.Logs() in the OnEnable because I am a man of deep talent and skill.
What I discovered was that this is just an unfortunate case of ScriptExecutionOrder and SuperTextMesh happening to run before the Mask. Upping the SEO on SuperTextMesh solved the problem and everything's fine. So if anyone else has a problem where things are having a hard time dealing with Masks, there ya go.
As to the outline, yeah, I figured it was URP. Sadly, I love those 2D Lights and how much time they save me. Interestingly enough, STM works perfectly normally in-game, the preview problem is limited to the editor, which is such a minor issue as to not be worth caring about for me. I'll keep that script handy in case I run into issues later on.