custom settings for trekking

Sorry I can't help with your first question, you probably need to explain a bit more what you mean by "retaining as much autonomy as possible". 

For your second question, accessing a map from watch mode just to see your current position:

Press the top right button, (as you do when choosing an activity), scroll to "Navigate" and select it, then press the Up button to get to "Use Map" and select it.  That will display your current position on a map, with zoom & pan controls. You can also move around the map with the touch screen, assuming you have Touch enabled. To get back to watch mode press the bottom right button 3 times.  

To make this more convenient, you could reorder "Navigate" to be at the top of the list of Apps. Long press the middle left button to get to Settings, scroll to Activities & Apps and select it, scroll to Navigate, select it, scroll to Reorder, select it, and then position it where you'd like in the list of Apps and select it.

There may be other ways to achieve it, but I hope that's helpful.  

Not sure if I understood the question correctly...

You can just start a hiking activity without a navigation-route. There you can access the map via the activity faces. Then pressing the "back" key will switch to the watchface, pressing it again back to the activity (with the map).

You could reduce the button presses by assigning a hot key to Navigation.

Hold the middle left button to get to Settings. Scroll to System, then Hot Keys, then choose whichever button or button pair you’d like and select Navigation to assign it.

But I don’t know of a way to get to a map view from the watch face (without an activity) by a single button press.

Regarding your other question about tracking with “autonomy”, it sounds like you want to maximise battery life. Expedition mode is designed for that and will create a track. I don’t use it because the track points are recorded at 1 hour intervals by default. You can change the interval to 15, 30, 45, 60, 75 or 90 minutes. If 15 minutes is not sufficiently frequent then you could use one of the regular activities (eg Hiking, Running, etc) and set recording to UltraTrac mode. That can record a position every minute. If that’s still not sufficiently frequent (and I personally never use it) then you’ll have to look at extending battery life by other means. That could be by reducing power consumption or recharging while recording (my preferred method for extended activities).