Is it possible to disable rendering features all together to optimise for mobile?

It depends on the feature:

Lights will consume resources only if the light exists in the map and its being rendered. A dynamic light with no shadows consumes little to no resources (on mobile by default you can only render 4 dynamic lights at all times).

You can disable most mobile visual effects by setting “Enable Mobile HDR” to false in Edit > Project Settings > Rendering.
There you can disable most processing effects too in the default settings section.
I recomend you to look into the scalability documentation to see how you can further remove unwanted effects and features: Scalability Reference | Unreal Engine Documentation

Is it possible to disable things like lighting, reflections etc alltogether? I’m not using any of those features but I was wondering if it takes some performance, no matter how little, to have them exist in the game’s code in one way or another.

Thank you, that’s a really good read. If all my materials are unlit, is there any point in trying to disable / remove engine features that are not used by my scenes / materials? Does the logic for reflections and various other things get packaged into the app, increasing the size?

If all materials are unlit the game will only contain those shaders when packaged. When packaging a game only what is used is included.

EDIT: there is a special option you can set so the game will not contain editor conten in Project Settings > Packaging > Advanced (arrow pointing down): “Do not include editor content”.

Take in account that if you use editor content in your game it might crash when the asset is requiered. You can also set “Include Prerequisites” to false so the game does not include other installers (might be needed for your game to work properly, so make sure of what you are doing).

Both options will reduce the size of your app. Please remember to mark this as the correct answer if it satisfied your doubts so other users can find it easily.

Make it a great day!

Just to make sure, what is editor content? Does it include custom material expression nodes such as 3way lerp, etc? Or is it just the basic sphere and plane etc. Do prerequisites matter in a mobile game?

Material expresions are not considered editor content, but any other asset that is not from a content pack (aka starting content) is considered editor content. I don’t know about Mobile Games prerequisites, my first thought is they don’t, but you will have to experiment with it.