When did you move to fully prone tummy time?

Baby is 9 weeks and regularly gets inclined and supported tummy time on me or her dad. She's pretty engaged as long as she's upright enough, but if we lay her more flat, either on our bodies or her play mat she just lies there in a face plant (or sometimes starts rooting. 😅) I've tried supporting her with a pillow or tucking her elbows for more leverage but it doesn't seem to help.

Should I just be letting her face plant for a bit? Is there a technique I'm missing? Is it too early to be putting her in a fully prone position? Am I just being an overanxious new mom?

ETA: At 10 weeks she can suddenly lift and hold up her head from a fully prone position for, like 5-10 seconds. Babies are wild. Thanks for all the advice! I am a proud momma. 🥹