Subtle distinctions in font weight or italic-ness? Variable fonts give a lot of options.

Mathematic notation uses subtle tweaks/additive glyphs to the same characters all the time.

A different spectrum per type of colorblindness. It should work fine, unless you're completely blind to all colors, which is a very small proportion of colorblindness, most are only lacking perception of one out of three colors.

You can already do that: in the settings.json under workbench.colorCusomizations, change editorBracketHighlight.foregroundX where X is the level of bracketing you want to modify.

In practice, though, the reduced color vision combined with the small target that is a bracket (color vision degrades for smaller objects), means that this is of limited usefulness.