When my baby begins to roll or their startle reflex stops and it’s time to stop swaddling, what should I do instead for their bedding? Is a sleeping sack safe or should they be put in a cot with nothing but themselves?

How old is your baby?
Is your baby making any attempt to roll or rolling yet?
How do they go with tummy time when awake?

Once baby is rolling, swaddling needs to be stopped & baby needs to be moved to their cot.
Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

A sleeping bag is fine, as long as it fits properly, especially around the neck & arm holes & has no hood.
Arms need to be free.
https://rednose.org.au/article/what-is-a-safe-sleeping-bag

A blanket can still be used as long as it is not loose or heavy, so it does not cover baby’s head & face.
Many parents will add another layer of clothing to baby (as needed) once they are rolling, as often baby does not stay under blanket & might “kick it” loose creating the risk for head/face covering.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep