A toggle to make whole UI disappear would be quite easy. Find the list of UI you want to make invisible and add a toggle button which toggle their visibility.
To make it immersive would be quite harder you would need to get them visible as soon as some events are triggered like being damage for your health to appear or using mana...
Easy but quite long work.
Edit: about binding it to a key, that would be more complex, the game do not have an easy way to do this. Ikkyo had to create a new skrit function for it and I did it too for DS2. But dsdll can be injected only with the mod executable. Sadness...
A toggle to make whole UI disappear would be quite easy. Find the list of UI you want to make invisible and add a toggle button which toggle their visibility.
To make it immersive would be quite harder you would need to get them visible as soon as some events are triggered like being damage for your health to appear or using mana...
Easy but quite long work.
Edit: about binding it to a key, that would be more complex, the game do not have an easy way to do this. Ikkyo had to create a new skrit function for it and I did it too for DS2. But dsdll can be injected only with the mod executable. Sadness...