I think you confuse security and privacy. Samsung are pretty secure from what I saw. They are not private at all however. You can debloat it with adb and maybe tools such as Universal Android Debloater.
There is a couple of guide such as this one: https://linustechtips.com/topic/1402380-ultimate-samsung-oneui-debloat-privacy-customization-guide/
I don’t have that comparison but here are some resources that critics FlatPak’s misleading security
https://whynothugo.nl/journal/2021/11/26/the-issue-with-flatpaks-permissions-model/
You can buy the SMS verification for Telegram on sites likes smspool.net using crypt I currencies such as Bitcoin or Monero.