We are known for growing extremely healthy Hoya plants. Our hoya soil mix consists of the following:
1 part well drained potting soil
1 part orchid mix
1 part perlite
Add some Charcoal. It increases the soil's ability to hold onto nutrients and beneficial soil microbes by reducing the leaching of nutrients from watering. It also increases drainage and allows air into the soil.
Just when you think your mix is chunky enough, make it chunkier. You can also add hydroton balls and worm casting. Your hoya plants will love it!