Stapeliads including Huernias can be tricky, too wet they rot, too dry the roots die back after which it can take ages to reroot. I have Huernia zebrina "magniflora" and have found that it grows best in the house rather than greenhouse. It needs a certain amount of shade, I found sunlight in a South facing window was too strong but in a North facing window they will go leggy. South seems ok during late Autumn to Spring.
They need a freely draining compost and, although you say your compost is freely draining, I think the peat based mix will be too water retentive. I would use a loam b