aiming the leather strap through the cheek protections ring.

I think most likely they would have slung during sieges. During battles they had their mercenery slingers,archers,war machines,and everything was very disciplined,and standardized. The sling wasn't. So I think some might have used the sling when they weren't given an order of tight battle formation. Can't think of many occasions besides maybe sieges..

It is known that slings were used in the conquest of Constantinopolis in 1453. Ottoman sultan Mehmed II was reported to have positioned the slingers and archers personally. This siege was famous with the extensive use of cannons, gigantic cannons appeared in the battle scene for the first time. Interesting that such a primitive weapon also played a role together with the modern technology of the time.

Guerilla warfare was anything but Roman.  Ship to ship bombardment..not sure how many legionaries would have attended something like that.

very neat - but seeig you in the armour really opens up the weakness of legionnaires to slings.

large round stones, slung along the floor - break ankles, knees, toes.

zero leg protection

C_A ,last weekend in Hungary,Savaria,i reecreated what i've been long thought,would be an excellent long range tactic in pre firearms ages.
To throw,stones,loose arrows on two different planes: lobbing high arc and relatively flat,by two groups of ranged arms.

So besides a roman testudo,this could be devastating,as most barbarians would fight mostly individually,raise their shields as their own reaction and not as a trained army to a command.

I was fighting along with some 3-4 century late roman army,against normann warriors. The battle started with the romans besides me throwing a hail of plumbata's in a high arc into the lines of the normann/viking reenactors bout 40m away. They raised their shields high to protect themselves from the falling plumbata's ,which were falling in a very steep angle.
Meanwhile,i was zipping tomatoes on a flat plane to their feet,which was totally exposed,and unprotected,up until their waist. I had only 4 tomatoes for that battle and scored 3 hits on legs, while on of them after he got hit, was like dancing,raising his legs like lizards on hot sand.
So in case i would have used solid rocks,that would have been 3 normanns with full armor,shield,helmet,chainmail,armscoat,arm protection with broken legs, in 15-20 seconds. By only one slinger.
And no matter how good a soldier you are,how well equipped you are, with a broken leg,you ain't gonna fight .
Maybe even occupy 1-2 other soldiers,that would drag you behind the shieldwall,before the charge, that if you really deserve it,and i doubt,that much of that would have happened just before a charge.

It felt great that a tactic that once popped into my mind has worked so well in reality ,well reenactement,but besides using tomatos instead of stones and no broken legs,it worked like a charm.