Anything else I should do before planting in heavy clay?

Check drainage before deciding how much extra soil to add. A simple hole filled with water can show whether water is sitting for hours, and a soil test can flag things compost won’t fix. If drainage is slow, avoid making a thick new layer over an untouched, compacted base. Blend the layers together where you can.

I’d loosen more than just the surface six inches, especially if the clay underneath is packed hard. Rough up the base and sides of planting areas so roots aren’t faced with a smooth, solid boundary. Don’t dig or tread on it while it’s really wet, though, or you can compact it again.

You may not need to remake all the soil if the plants suit the conditions. Some plants tolerate clay well, and adding compost gradually with mulch can improve the bed over time. Pick plants for the actual drainage and sunlight you have, rather than assuming every bed needs a deep layer of bought soil.