My DD is 11 and an only child so I know where you're coming from. We have a rule of no devices Monday to Thursday night, luckily that was suggested to me by her best friends mum so she accepted it pretty easily knowing she wasn't alone. She could use her phone to check messages and emails, but no watching or playing anything. We do the same (until she's gone to bed at least 😄).
She doesn't finish school until 5pm and then she's usually in a sports club until 6pm so by the time she's come home, had dinner, had a shower, sorted out her bag for tomorrow it's not far off bed time anyway. She can use Alexa so she'll listen to an audiobook, music, or practice her instrument with free evening time.
Friday night is movie night for the family so while that's screen time it's also family time.
Weekends she can use her phone and play games all she wants but I let her know on Saturday and Sunday roughly what we're doing that day, if we're going out somewhere and tell her it's up to her how she manages the rest of her time. We try and meet up somewhere with family or a school friend just to get out the house.
Also, not all screen time is the equal. I really don't mind her playing games like Fortnite, especially if it's in a squad with her friends, it's great to hear them all chatting and organising themselves. Watching video games is another story, I can't stand these YouTubers and when I see it I'll try and distract or ask to play a 2 player game or something with me.
...and I didn't see you mention it but if she's on social media then absolutely cut that out and block it from the router. There's never a time for that.
Tldr: restrict school nights, sign up to after school clubs, fill up the weekend with going out, let her play in any time she finds in between. No social media.