Just keep i mind where these plants grow in nature. Deep, organic loam under tall tropical trees. Make it like that, or as much like that as you can. With that in mind, cactus soil would be the opposite of what you want.
In Salt Lake City where we lived before moving to FL, there is also deep, sticky alkaline clay. Both my kids still live there and we made a garden out of a baked clay moonscape at my daughter's house a few years ago with the addition of at least 20 pickup loads of compost. Adding sand to clay soil just makes it behave like concrete.Only adding organic materials reduces the clay's tendency to stick together. The expansion and contraction of the organic fibers opens air spaces in the soil.
You should also consider the pH of your clay soil when you are amending. I'd be willing to bet your soil is on the alkaline side (high pH) and your Philo needs neutral to acidic soil. If you plant it in a hole in the clay filled with organic amendments, it will do well until it gets roots out to the alkaline clay, or the clay filters into the picture. From then on it will struggle.
I'd still go for the half barrel option.
