We just built a bunch of 4x8 raised beds this year, but used them for other plants, so I'll be doing sweet potatoes there next year for sure. We do a similar mixture as you, with 1/3 compost, 1/6 medium perlite, 1/6 large vermiculite, and 1/3 coco coir (environmentally friendly alternative to peat moss).
The clay soil area had some of the mixture above, plus local soil, plus amendments from last year.