The alternative would be to run the JSON through e.g. jq/sed and make it dump out a chunk of C++ that would create an object with proper fields and subobjects.

The alternative would be to run the JSON through e.g. jq/sed and make it dump out a chunk of C++ that would create an object with proper fields and subobjects. This C++ code will have about zero template chicanery; instead, it would just call constexpr constructors which, I imagine, would be entirely boring — this C++ code will be compiled much faster.