My mix is 5 parts fine redwood compost, 1 part potting soil (2/3 potting, 1/3 local), and 1.5 parts coarse Turface, because i messed up and spilled it when pooring.

The potting soil is Kellogg's Patio Plus. The redwood mulch is pretty darn fine compared to some pictures I've been seeing, with a lot of dust and small particles. I added a small amount more potting soil and redwood to make up for the 1.5 of Turface. I don't figure slightly too much turface will be a problem when I am aiming for hot weather tolerance.

On top of fertilizer, do I really need to worry about lime if my water is hard and has loads of calcium and other minerals?

The quick answer is, No, you don't need to include the slow-release fertilizer.

That said, let me give a longer answer. If you suspect that you might not fertilize
as regularly as you hope, then you might consider mixing in some of the slow-release.
I use Osmocote 4-month because I'm not as consistent with fertilizing as I should be.
With a 4-month formula, I can also be fairly certain that the fertilizer will be gone
by the end of the season.

In fast-draining soils, these slow-release products don't last as long as advertised
for the simple fact that we water more often - which has the benefit of flushing the soil.

Josh

4/6 plants are repotted and looking happy on day one. Just the pair of Habs to go!