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/

FlatPak is not the as secure as everyone think it is.