Don't know what the problem is.  I have never seen with dozens and dozens of spinning outfits line jumping up hitting the rod shaft.  Started off with 14#, if anything would be a coiling problem.  Even if it is doing this, do you know it's hurting performance, you haven't landed a fish or even used it to fish with. Test it in real time, you may not even have a problem.

I just spooled the 2500 reel with 12# fluorocarbon, and put that on my 7ft st croix. Still a lot of line slap. And this rod is rated 7ft medium, 6-12lb, 1/4-5/8oz. Casting a 3/8oz plug to test it... line slapping like crazy

Woah ! - 12lb. FC is too big to spool on a 2500 series reel and will spring off making the line hard to control ... You need to keep the FC line to 8lb. or less

If I understand your line slap occurs reeling in line, not casting out and this occurs while testing dry, not actually fishing in water.

I wouldn't be concerned. Spool the smaller diameter 8 lb line and go fishing. Wet line behaves differently than dry line, that is one reason a line conditioner is important.

Tom

Braid does wonders if you haven't tried it yet. Thick mono does not get along well with your average spinning reel, and going over the recommendation can lead to tons of problems!

use braid and don't rapidly reel slack line. You just went from a line that was too heavy to one that is too stiff. Remember line twist doesn't affect braid and neither does line slap, frankly. Sometimes I get that when reeling quickly to take up slack caused by the wind and I don't have any line mgmt issues as a result.