My boy does the same too, even till now 5.5months but rarely at the moment. Since his first month he absolutely hates being swaddled so we replaced it with constant contact sleep and cuddles which worked. I suppose he also felt safe in mummy’s and daddy’s arms. You could definitely try that if your LO is still around the 3-4 months age. My boy now prefers his crib hahaha. So when it happens we use bean bag like you did, and also just gently tap his bum bum and tuck his hands back. But we did observe that more or less he will slowly soothe himself back to sleep without the need of our interference too 😯 so I guess theres no deciding when your LO is ready to be unswaddled as all babies feeling of comfort is different ☺️
Dont suddenly go cold turkey on your baby. He not only get startled reflex but also unfamiliar with not being swaddled and get tense. If he has always been swaddled, then slowly unswaddle him by leaving out one arm at a time. This will reduce the startle by half and keep changing hands until he is familiar with not being swaddled. There are those loose swaddles which you can transition them into - the sleeping bag kinds.
LO currently 1month old and hates to be swaddled. So we didn’t swaddle her and she sleeps freestyle which she really loves. When we swaddled her, she would make grunting noises, cry, etc. Indeed there were times where LO got startled a few times and woke up but eventually she slept back on her own.