LineRender width not consistent across resolutions

This sounds wrong. Scaling down even though the resolutions mentioned are larger AND smaller than the reference resolution? This is from the docs and should help you figuring out how to calculate the right width:

The Line Renderer does not render one-pixel-wide lines. It renders billboard lines (polygons that always face the camera) that have a width in world units and can be textured

Try it for yourself! Create a Canvas, Create a game object with a line renderer, turn off "Use World Space" on the LineRenderer and see that if you switch between resolutions, you get various widths that don't really correspond to resolution sizes in a meaningful way (or at least that I can tell!).

It's strange that something as basic as line renderers have highly variable line widths at various resolutions. Anyone know how this is calculated?

Why use line renderer inside canvas? Line renderer is more for 3d space and it's not a sprite that can adapt his properties acccording to canvas.