I would vouch for Thunder for Android and it has iOS version too. But I don’t think it can override the system font, so it is out of your requirement.

I only know two desktop apps on Linux, both of them are available on Flathub.

• Interstellar
• Lemonade

It looks like only Interstellar is still maintained, Lemonade last update was a year ago.

Love photon. It is quite usabale as PWA too on Android, though I use Thunder instead.