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://hanako.codeberg.page/

https://flatkill.org/

https://whynothugo.nl/journal/2021/11/26/the-issue-with-flatpaks-permissions-model/

https://alternativeto.net/list/28655/the-ultimate-f-droid-setup/

You can download them from other sources ;)

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