”[the CBP agent] said if I didn’t give them my phone, my interrogation was going to take much longer and they could retain my phone indefinitely”
I would take a punt at filing a complaint about that. Threatening someone with selective enforcement must fall short of professional standards at CBP, as stated on their .gov page:
https://www.cbp.gov/sites/default/files/2024-09/exhibit_08_-...
It's funny you think that CBP has any real professional standards that any of their agents would possibly be held to if it were inconvenient for the agency, at least under this administration.
They’d probably say it wasn’t selective enforcement, because the phone scan is a faster way to complete their investigation compared to an interrogation. It’s the same disclaimer used for a manual search when you opt out of the body scanner.