Good points. In my case I decided to make my own mostly because I want to dry aquascape and take my time. I intended to fill with RO first as well for the leak test (IMO if the tank leaks I’ll have a much bigger problem than wasted water). However, I did recently mix salt in my 20g QT and you’re right; it was really difficult to get it all to dissolve which was easy to see with a bare bottom. I wouldn’t want pockets of salt on the sand or rocks. Here’s an idea though; mix in the sump with the return pumps off. Then once fully dissolved mix it with the rest of the tank and repeat in batches.Why not make the RO yourself? With that big white storage tank your getting it be easy to let it run half a day for two or three days. I would not trust someone else's RO for that quantity, I would want some fresh filters.
I would also suggest not mixing the salt I side the tank. It can be hard to dissolve it all that way and you could end up with pockets of salt all over.
Finally I would fill the tank with tap water or your RO waste water for leak checking before filling with your final salt water, just incase you have a leak that requires draining, and also to wash away any last bit of manufacturing and shipping chemicals.
Sorry if I am overstepping here, I think you may have been doing this longer then me, these are just the thoughts I had when you said LFS was bringing RO water.
Full disclosure my first tank was filled with the saltwater from the LFS that had been in it for years already. My second tank I filled myself with homemade RO/DI mixed with salt in 5 gallon buckets, heated and aerated for 24 hours.
I don’t want to leak test with 200g of intentionally wasted water, but I don’t want to do it with salt water either. But I guess the other side is that beyond leaks, there’s a chance the leveling could change with the extra weight too. The DI waste water is a decent idea but I don’t think I could make that work practically. Quite the conundrum.