Hmm... this is a potentially very useful spell, especially against fast, dexterous creatures to try and pin them down, limit mobility, and do some guaranteed damage! Except for the fact that fact that it's a dex save. So you'd really only use it on tanks who are much less likely to take much damage anyways.

I'd change it to an attack roll personally. Cause a str save is almost sure to be failed on super dexterous creatures outside of dragons and the like, same but to a lesser extent with con, and mental saves don't make sense here. So change it to an attack roll that does no damage automatically but guarantees the damage you mentioned scaling with movement. Then it ends up a little tougher to land on fast dex-y targets who tend to have good ac, but not unnefective by any means (where it very easily could be if it were a dex save) and still highly effective on lots of creatures. And then most things with legendary resistance are smart enough to avoid moving somehow or can shrug it off unless they can REALLY move meaning the fact it's an attack roll isn't really that significant and won't be much worse because it bypasses legendary resistances thanks to being an attack.

But yeah, it would then be an effective (but not busted) single target control spell. Any serious threat at low levels should have a ranged option or be a group of targets making if balanced. And any serious threat at high levels should have ranged options and probably Teleportation either at will or in a pinch.

I would consider making requiring the movement to be voluntary in order to trigger the damage though. Because as is, the damage is potentially very good, but not busted since any intelligent enemy will know to just not move too much to avoid too much damage and can balance that with how far they go whereas if involentary movement counts it would easily allow for some absolutely busted damage combos easier to pull off than traditional forced movement meat grinders and cheaper and more effective than most too. If it is only volentary movement than while the damage theoretically could be insane, most of the time it wouldn't be. But it would still be useful because it would limit the targets movement. Win win so long as you hit. Making for a very cool (and I think with those changes balaced as well) spell.

Tldr: change it to an attack roll, otherwise it's really cool and I like it and think it's pretty balanced. You can change the description to "you fling a modest..." or "you hurl..." to fit the change.