I don't know all the reasons behind using curl and all GNUnet's requirements, but have you guys thought about switching to wget2? It is a GNU package and has a nice library (libwget). It supports GNU TLS natively, it is supposed to download faster than curl, and if a minor feature is missing it might be an opportunity to make libwget grow.
A comparison table (by curl): https://curl.se/docs/comparison-table.html --madmurphy
