do you have a shut off valve so you can shut the water going to your new main dt? you can always just fill the new tank with ro water and mix the salt in the empty tank and once the salinity matches then just open the valve for a min or so then shut it off. do that over and over until you get things slowly mixed together. like you said it just like doing a large water change. spread it out over a few days
I'm gonna have a dedicated pump only for the DT. External so while I will have ball valves to adjust down a bit if needed I can just turn on and off the pump. All I would need to be concerned with during that time is drain back which may be substantial due to the length of plumbing so I'll have to keep an eye on my skimmer during that time.