We read about average ages for milestones, but every baby reaches these at different times. And it's easy to freak out...so unless your daughter is way behind, there's really no need to worry. And your daughter is not way behind. My son rolled over at 5 1/2 months! (And was crawling, belly off the floor, at 6 months. Early!)
Listen to your gut, here. Your little one sounds happy and healthy, so she's 99% likely to be just fine! But you seem worried about her tummy time. So just add in more- and it doesn't need to be just traditional tummy time. Lots of babies hate that anyways. It's really all about neck, back, and tummy strength.
You can:
-put her in a bouncer/jumperoo or exersaucer, and that will definitely get those core muscles strong!
-prop her up, belly down, on a Boppy pillow (or rolled up towel). Easy to do on a bed or rug. Put a cool toy in front of her- something with buttons and neat lights and music.
-keep having her sit up (like you're doing). Help her prop herself up with her hands, etc. Lean her back on you.
-help her stand, by holding her under her arms.
And in all honesty, she's going to practice rolling in her crib once she's ready. Waaaay more than you'll want her too! :) Your little one is fine. She's normal, and so are you for being all concerned!
And at the next doc's appt, ask your pediatrician about all this stuff. It's his/her job to let you know what you really need to worry about!