Your saying you have a pump in the ro tank? If so you can mix your salt in it and leaving it mixing for a couple weeks if you need too, it will be fine but no pump on mixing it can/could be an issue depending on the salt mix itself and if there are organics in it.
Run the pump overnight to let the salt mix well and then let it sit as long as you want. You don't have to keep the pump on. Even if there were organics in it, which there shouldn't be since you are using RO water, moving it around isn't going to change that. Stuff will rot whether water is moving or not.
Now, if you are trying to keep it heated then it helps to circulate it but if not, don'y worry about it.
Just FYI, I always have 30% on standby mixed, heated and being circulated, usually for two weeks, not entirely sure but I think I have had some precipitation of calcium without it circulating for extended periods but I assume every brand is different