This is what I use now to build things for tex4ht
make4ht -u -c ~/nma.cfg -e ~/main.mk4 index.tex "htm,pic-align,notoc*"
To split the document, say 3 levels, then the command is
make4ht -u -c ~/nma.cfg -e ~/main.mk4 index.tex "htm,3,pic-align,notoc*"
Make sure to have the main.mk4 build file in my home folder as well as the .cfg file. The main.mk4 I use now is written by michal.h21 main.mk4 and my .cfg is nma.cfg
You need to have latest make4ht installed, which does not come with texlive. You can get it from https://github.com/michal-h21/make4ht
see how-to-speed-up-tex4ht-image-generation-process for reference
Thanks to michal.h21 support for making the above possible.