How do I render part of one texture into a larger texture?

[CanyonTurtle]

I want to port an existing pixel game to bevy. My existing game will work provided that I provide a function blit_sub which draws a sub-rectangle of a spritesheet to the screen at a specific XY location. I know this seems quite anti-paradigmatic, but if I can make this work, then Bevy can essentially compete with a raylib-ish experience.

My current ideas of how to approach this, and their roadblocks, are

1. Copy a subset of one image to the pixel-perfect "canvas" Image (see the pixel aligned example) somehow. Problem is, I can't figure out where to look for this, and I want it to work from GPU textures.
2. A shader. Problem is, I don't know wgsl (yet), and I'm not sure if learning this is the easiest way to accomplish what I would want.
3. A heap of sprites frim a spritehseet that handle successive blit_sub calls. This just seems memory intensive.

Note that the actual image data doesn't need to pass through my blit_sub; just the XY and rect bounds. The actual spritesheet is assumed to be peesent in bevy.

Any advice?

[CanyonTurtle]

Reading an in-depth wgsl shader tutorial is helping.