The old way was a 5:3 ratio of MgCl to MgSO4 when using two part and is super cheap to make on your own. It is hygroscopic so you'll want a tight fitting container.
I use it since I got a ton cheap but will be moving away from it since it leaves a brown precipitate/scum. It definitely gets warm and you need to have good water motion or it creates a brick on the bottom. Mixing 20g in 8fl oz like the instructions say.
@Ayman got burned with the same issue when he got his second hand RODI. Always swap membranes and resin.
Edit: I also don't like these systems where you can't see your resin since you don't know what's exhausting first.
Any point source light will have shadows, that's unavoidable without like T5 or blade style lights. But honestly with your caves and elevation changes, it should look great and you can adjust it forward or back depending on how it looks. Plus you'll get less algae growing on the glass.
I'd stick with what you have unless the lenses are burned out since you didn't have a lot of lighting requirements with that livestock.
Otherwise the Kessil a360xe tuna blue would work well and it's on sale currently. I use that on my clams in the same size area, but it can be dialed down...
Definitely leave the pump on, otherwise the chamber gets stagnant and things will grow. It does need cleaning once in awhile, especially if you don't have the cannonball.
Are you leaving your plank pump on 24/7? I found that this is pretty critical to keep everything from getting gunked up in the cylinder. The mysis should fully rehydrate if you adjust things well. But having mixed foods like Derek said can prove problematic.