Coco peat will hold too much moisture, so it will understandably rot the base, there are no roots yet able to take in moisture. Use more porous media, like cacti mix and add more pumice or perlite to make it much more grittier.
I do not want to use sand either. better pumice, perlite or crushed lava rock with the media, and putting them around the base of the cuttings, as top dressing so it further protects it from sitting in too moist media.
You are trying to encourage root formation, so it needs more warmth at root zone but not too much sun yet that may dry out and stress the cuttings too fast.
