Other ROMs are usually very bad at security with many being late to secuity updates, not supporting bootloader relocking, and not using the secure element. I would not trust them in the hands of someone who isn’t tech savvy or is at high risk of hackers.
That is all true, however it seems like a slippery slope to me.
To stop scams, it would instead be a good idea to block app installation (of ANY apps including in the Play Store) when the screen is being monitored or a call is active.
Then when sideloading apps, grey out the install button for 3 seconds to hopefully pull the user out of any mindless flow state a scammer has put them in.
The solution is GrapheneOS. You can install it on Google Pixels today, and Motorola is going to release some devices with official support for it in 2027.
Basically all of them use the “Cast” feature, so you just need to detect that.