@HuniSenpai I could see that being helpful but it's not exactly what I'm saying.
First, an Isometric grid would be 30° angles, not 45°.
Secondly, it seems like it would be most flexible if it aligned to the grid angles that you set under View > Grid and Axis Manager. Under the default set up, this would lock your brush at 0° and 90°, and maybe 45° could be added here as well. But if you set up an advanced grid, like an isometric grid, it would lock to those custom angles.
Another option would be to add a checkbox that says "Lock brush to grid angles" and leave the shift functionality as is.