3.5mm is an audio source, USB is a data source. Any headphone with a USB plug also has to convert digital to audio, something your phone already does. USB is not a replacement by any means.

It very much depends on the pair of headphones and on the hardware of the phone.

My LG phone is a beast. Sure, it’s not quite the same audio power as a headphone amp would give, but the audio quality and convenience is fantastic.

It still degrades audio quality and that’s an area I refuse to compromise on.

The smartphone is not the expensive part of my mobile audio playback setup, I expect it to be compatible with the standard playback interface of wires.

My headphones (wired) cost more than a smartphone and I expect my handheld device to be compatible with them. I’ll rule out buying models based on this feature.

Really? They’re much better audio quality than bluetooth.

No, the need is still present. Headphone jacks are pretty essential still, wireless tech is not a replacement.

Let me introduce you to open street maps. https://welcome.openstreetmap.org/what-is-openstreetmap/ or more simply https://www.openstreetmap.org/

Maybe consider starting to play with Darktable. Get a sense if it is a viable alternative to support.