What model s20 have you got? North American Qualcomm-powered models are essentially unrootable.
Yeah, you'll read about apps that claim to be able to recover photos. All I can say is that it's very hard to find reports of success from people who are independent of the app developers.
To make it worse, you didn't just delete the photos, you reset the phone. The problem is that the internal storage is encrypted, and the reset also deletes the encryption key. This is good design as far as data protection goes precisely because it makes recovery practically impossible, but in this case that's very bad news (I say "practically" because I'm never going to promise that if your device was important enough for the NSA or GCHQ to throw their full resources at it there wouldn't be a way, but in practice it's beyond the capabilities of any ordinary user, even a computer expert).
