I'm not sure whether I already asked about this in the past ...
Currently SHTOOLS allows extraction of harmonic coefficients from DH grids up to (roughly) half the number of iso-latitude rings. However, there is a way of doubling that limit by means of latitudinal FFT resampling, which is, for example, described in Appendix A of https://arxiv.org/abs/2304.10431
This approach has been tested and validated in an astronomical context for some time now, and I was wondering if it could be of interest for SHTOOLS as well. It would allow increasing the band limits for both l and m by a factor of 2 on the DH2 grid.

(In addition, the paper above describes efficient spherical harmonic transforms involving completely irregular grids, which could also be of potential interest for geodesy. But that would be a different issue ...)