There is no need to use pool salt for faster dissolving. Even pellet salt, which does dissolve more slowly, still dissolves quickly enough that it is not a problem. I suggest crystal salt, but anything other than a large single block of salt will work. Simply brush any salt around to spread it out right at first and again after 15 minutes and it should all dissolve reasonably quickly. Leaving the pump on for 24 hours is a great idea, but four hours is probably enough (less if you pre-dissolve).
Water softener salt, often called solar salt, is the least expensive. You want the kind without any additives or special features, usually in the blue bags, avoid rust remover, iron fighter, and table salt! For example, Cargill/Diamond Crystal Salt Crystals for Water Softener or Morton White Crystal Solar Salt. There are other forms that will work, pool salt, pellets, etc, but the crystals are usually the least expensive (sometimes dramatically so).
Also, low water temperature can cause some SWGs to read the salt level as lower than it actually is. Some are temperature compensated and some aren't.