The alternatives considered:
Fantasy Grounds: too limited in free edition.
D&D beyond: has poor features for free, too.
d20pro: has only 30-day free trial.
hextml: an interesting alternative, but with hexes and looks like less feature-rich.
Some cool stuff that came cross along the search:
Play D&D for free (without registration and SMS): https://www.wikihow.com/Play-Dungeons-and-Dragons-for-Free
Topic on the github: https://github.com/topics/dungeons-and-dragons
Lots of tools: http://donjon.bin.sh/
SRD online: https://www.5esrd.com/