u/egypurnash's point was about limiting glyph complexity. You could canonically decompose then look for more than N (say, N=3) combining codepoints in a row and reject if any are found. Canonical forms have nothing to do with actual glyph complexity, but conceptually[0] normalizing first might be a good first step.

[0] I say conceptually because you might implement a form-insensitive Zalgo detector that looks at each non-combining codepoint, looks it up in the Unicode database to find how many combining codepoints one would need if canonically decomposing and call that `n`, then count from there all the following combining codepoints, and if that exceeds `N` then reject. This approach is fairly optimal because most of the time most characters in most strings don't decompose to more than one codepoint, and even if they do you save the cost of allocating a buffer to normalize into and the associated memory stores.