I hobby-code for DOS, partly because of nostalgia, but much also because it (specifically DOSBox, or DOSBox-X) is such a fantastic and stable virtual machine. No one is going to deprecate even some minor part of the API. And some emulators give you fun features like allowing any screen resolution to be set up to run in fullscreen on a modern monitor (exposed using standard VESA API inside of DOS).
There are some other retro computer or consoles that could probably be just as useful for this. But DOSBox (as well as QEMU+FreeDOS, for those that prefer that) are nice because they have fully open source implementation from the CPU-level up to the user utilities, so there is no need to mess with dodgy ROM downloads or such to get things working.
What do you hobby code? Can we have a glance at it?
Exposing a port isn't exactly a safe thing to do nowadays, and I'd be wary of the security posture of an "old phone". Proceed with caution.
Assuming you are using an old phone with termux to serve de webpage. What could be an attack vector?
The phone will be running Android 7.1