If you are dead set on updating to newer OpenSSH to fix this, then you need to manually compile OpenSSH and install it on those affected systems. This will not be trivial and is not easily documented here.
as for installing the latest packages, some apps on flatpak are community maintained or maintained by the devs themself example discord it's maintained by discord themself and it's the same version as the deb they provide.
As whether you can trust flatpak installing apps via flatpak are about as safe as the apps in your distro's repos. Meaning if you apt it will be the same as the flatpak version, if you don't want to use flatpak you can use ppa's they offer the same as flatpak meaning that it's the newest package instead of being a "stable" version like the one you find in your native package repo,
if you want a distro that has a up to date packages you should think about rolling release distros, rolling release distros are distros that as soon as a update is available for a package it's available for you and its in the native repos some of the most common distros that offer that is arch(based) openSUSE Debian Unstable (also known by its codename "Sid"), if you want something familiar since you are using ubuntu, I would recommended debian unstable as it has the same architecture as ubuntu, both being based on debian and use apt so you can easily migrate to debian unstable, if you are feeling adventurous, you could go to arch and try it to get to know arch and it's packages
dpkg
provides the PACKAGE VERSION, where as asking the app to provide version will provide the PROGRAM VERSION, ie. different results that are usually very close, but are not always.