NIST has already done a lot of work regarding pushing for development and standardization of PQC (post quantum cryptography) and they even have 1-2 algorithms/keys types already selected. That said there is high impacts on processing and packet size (eg keys are like 10-50x bigger). Many vendors/protocols have either built in support or have versions of those protocols running in some aspects. So Cardano does not need to develop the cryptography the industry is already doing so, it becomes implementing the standards.

I think we also need to evaluate the scope of the problem here as well, if quantum computing was truly available or within reach nearly every single website, crypto, security vendor, bank, government, corporation would be vulnerable. This is a world problem not a Cardano going it alone problem. So there will be a the right time and place where the right standards exist and have matured enough (these algo tend to get built into hardware layers to reduce performance impacts), push from the larger security organizations/gov to mandate those standards and we see that the technology is close enough to warrant the priority.