Welcome Oldute...
From what I have seen of the WA pea gravel, it looks like it has a reasonable amount of iron content, and I have seen it oxidise around the steel posts I have on my property.
This oxidation may cause you some issues, do a search on the forum on rusty bath tubs, and see what you turn up, I know some people had issues with rusting bath tub setups, and this may be something to keep an eye on.
Others have used blue metal, and there is a quarry in the hills, so cost is reasonable, and may be worth considering for the expansion (depending on how you go). Just order the concrete agregate type, as opposed to the road base, as it should already be reasonably clean. Nocky has used blue metal in his setup.
In regards to clearing up the water (and pump size), if you put a tpce on the pump outlet, and a valve you can put an offtake back to the FT, and create a whirlpool effect in the tank, this might help to swirl the water in the FT and separate out the cloudy material (likely clay). Also helps with salmonoid fish growth, of which trout are (a treadmill for fish

).
Those temperatures seem normal for this time of year to me for a fully exposed system.
What's the bottle jack in the pic for? Is they material too heavy for the stand and you need to prop the centre?