Inb4 she just eats food like Luffy and gets right back into it
The biggest issue is simple: Since Bonney is always using her Fruit to appear as an adult, it takes a lot out of her to then use strong moves on top of that. And, when she does, she uses up too much energy and reverts to her true age, 12. The only solution to Bonney getting much better at using these abilities would be permanently increasing her age via Shinobu, as then Bonney would be an actual adult, with much more energy/Haki.
Unlike with Tama, though, I just don't give a shit if Oda does a permanent age up with Bonney, since she could just use her Fruit to appear her younger age.
I do have Bonney showing up in Wano Act 4 (Bonney and Carrot would be good friends IMO, Carrot's actually only 3 years older than Bonney, and has a lot of knowledge on how to deal with loss and grief), so if Bonney gets a big power up outside of actual training/her Fruit, it will be Shinobu aging her.
Bonney also just can't eat as fast as Luffy because Luffy's Fruit lets him digest food at a stupid rate because he can stretch his entire digestive system. Bonney has plenty of weaknesses that keep her from being another Gear 5 Luffy.
I think you're right, though, and that Bonney can recover quickly through eating. I wonder if eating during Gear 5 extends the duration?