But is it actually inaccurate at scale?

I get that drawing a projection is inaccurate at scale, but if I do all my calculation as meters north/south and east/west of 0,0 on the equator, won’t all my distance calculations be correct?

Like 5.000.000 east and 0 m north is 5.000km from the origin. I cannot see how that could ever become inaccurate as I move further away.

Where is the inaccuracy introduced? When I reproject back to coordinates on a globe?

Btw, you can plug your comment into ChatGPT and it'll give you a reasonable answer. The short answer is: distortions.