They have poor space/bandwidth usage properties, which is one reason
why Bitcoin doesn't use them today, but as far as I know the same is
so for all post-QC schemes.

I believe post-QC schemes based on Regev's LWE assumption are getting competitive with more traditional schemes. A paper from 2010 says they were able to get to around the same as large RSA key sizes (2048 bits), which is much worse than ECC but not entirely infeasible. Especially given that barring some breakthrough, by the time QC is a real problem we'll have gigabit wifi and 32 core devices with a terabyte of storage embedded in our hands :)