Low-Input-Lag mode Monitor?

[Chief Blur Buster]

Where does Scanline Sync fit here?

Depends on where you place the tearline.
If inside the VBI, the S-Sync looks identical to VSYNC ON at the cable level and the panel level. You do have fewer framebuffers waiting in the GPU drivers though, less lag). It just looks like VSYNC ON.

Now if you put a stationary tearline onscreen, it will be different. It will take me an hour to create a new scanout diagram for that though.

Naively, I was hoping to use SSYNC get VSYNC OFF performance but with a consistent tearline some [comfortable] amount above my crosshair, to ensure that area has the lowest lag. Am I thinking about this right? Am I using the right tool for the job?

Thinking a little more critically, and after a little more caffeine, I think this would just be like VRR but worse. Consistent scanline location but visible instead of during VBI. With no real benefit.

Depends.

Actually it would be a shifted latency gradient. Each VSYNC OFF frameslice is a latency gradient, where lowest lag (during non-strobed operation) is area right below a tearline, and highest lag is area right above a tearline.