I don't doubt you. Linux font rendering has been good enough for so long now that its surprising when people say its worse than some other system but I think it is still a reasonably common complaint so there has to be something to it. A lot of distros probably don't have a very good font selection installed out of the box compared with proprietary systems and I am sure that plays a role.
My desktop has a 38" ultrawide and the pixel density is a lot lower than your dual 4k monitors so I want to do everything I can for font rendering and your post has got me asking questions. I am in the process of configuring a minimal, low distraction tiling wm setup for a bit of fun (also another nvim conf spring clean). I hadn't considered changing the font rendering defaults.
I think I have all the fonts you list installed except for Hack. Inter is also a good one for UI. I don't use Fira Code anymore for code/terminal but I keep it around. It is a nice code font with ligature support but it didn't have an Italic variant and I like subtle use of italics in code and docs. Currently using Iosevka for mono but next week it might be something different.
There is no simple answer. Its is almost entirely dependent on implementation. All systems are vulnerable to things like supply chain attacks. We put a lot of trust in phone vendors, telcos and Google.
If you are going to compare to something like termux you need to compare with an equivalent sandboxed environment on regular linux, like a docker/podman container with appropriate permissions. As far as I know they use the same linux kernel features like cgroups and namespaces under the hood.
Traditionally Linux desktop apps run with the full permissions of the user and the X window system lets apps spy on each other which is less secure than Android sandboxing by design. There have been attempts to do better (eg flatpak/flatseal, wayland) but they are optional.