6 mo old sleep question

My daughter is 6 months old and lately her sleep is getting worse. She is a stomach sleeper. We put her down ans she flips right over. She sleeps in a onesie, pants, socks, and a fleece sleeper. House is 70 degrees but she’s constantly cold. My question is what honestly should she sleep in? I’m afraid to get a sleep sack bc of her stomach sleeping. She can roll but ho in a sleep sack? It’s about to get warmer here so I’m questioning what she should be sleeping in..