Workaround for 20 Option Max in Multi-Select Text field?

I suppose one pretty elegant solution would be to have a pipeline ensure that the parent record always has the 50 state children records and have your users use Grid Edit on the embedded report to select the states they want. So they would not be adding the states one by one, the states would already be loaded and they were just checkbox the ones they want.

You could also consider having a URL formula button, which would either check them all or uncheck them all to deal with situations where the user wants either all 50 states or almost all the 50 states in which case they could click a button to check them all and then deselect the ones they don't want.

There are elegant ways to do this with a URL formula button, so that the response on the screen would be instantaneous, the user would click the button and the page would refresh with all of the check boxes checked or all of them unchecked.  

This Mass checking or unchecking can also be done with a Pipeline where maybe the set up is a little more intuitive but the response on the screen is not quite instantaneous. The user would probably have to refresh once or twice to see the effects of the pipeline running.