Is it true that Telegram doesn’t encrypt group chats at all? Maybe that would get their attention?
My biggest criticism of Telegram (but not the only one) is that they use homebrew crypto. Of course, I don’t know if your family would understand why that’s bad.
Are you sure it can’t be unlocked?
https://xdaforums.com/t/guide-to-root-galaxy-s22-plus-b-e-n-0-unlock-bootloader-and-flash-official-firmware-noob-friendly.4404351/
Many phones that don’t officially support unlocking can be exploited to do so anyway. Some will lose relatively minor functionality in the process (camera enhancements were lost on mine, but the camera still works fine) but the tradeoff is often worth it.