Don’t care too much about the supposed hardening, but it’s on FDroid and has UnifiedPush, so I use it over Signal

I would be

Chinese teenagers would be much more competent at running tech companies

Backups are, first and foremost, your responsibility. It’s unfortunately not realistic to expect someone to diagnose whether an issue is software-related or a hardware failure on any obscure DIY OS you might have installed. But as long as it’s possible to flash back the original firmware, warranty should still apply