So I want to add lightmaps to my mobile game, so the surroundings will look better, but I want to know what performance on different devices my app will have.

Is there a simple tool where you can at least simulate how your app will work on multiple devices. So that I could say: "Hey, test the performance of my app on iPhone X, Galaxy S8 and ASUS ZenFone Max Pro and give the appropriate performance for them". Because I don't have dozens of smartphones on which I could test it.

I would also love to hear suggestions for any mobile performance tools that are easy to use, or maybe even have some integration with unity.

Keep in mind that opengl emulation is very poor and unstable, it won't give you accurate result

I myself don't know of any simple tool for this. There are companies you can pay on the net to test your app on a bunch of different hardware devices, so you could make a build with some performance logging.

There are also emulators where you can very roughly match some specs to a device, but it won't give you much if any idea of performance in the real world, as under the hood they might be working quite differently to the actual hardware. Emulators are more useful for testing things like different screen sizes and RAM, and general bug testing, rather than performance.

Even more important than performance is the knowledge of whether your game will run AT ALL, and shader inconsistencies between devices. This should be less of a problem if you use unity as they do a lot of the compatibility testing for you, especially if you stick to standard mobile shaders.

In practice if you have a few really old, low spec devices, and you get it running good on these it should run better on newer devices. The biggest issue is often fill rate which mobile devices are bad at, and understanding that you are probably using a tile based renderer which have certain quirks. So if you select simple mobile shaders in unity and limit overdraw / transparency this will help a lot. Baked lightmaps is a good shout as they improve the look a lot and are pretty cheap hardware wise.

https://docs.unity3d.com/Manual/MobileOptimizationPracticalGuide.html

https://bitbar.com/optimizing-graphics-performance-with-help-of-real-mobile-devices/

https://www.construct.net/en/forum/construct-2/how-do-i-18/optimizing-a-game-for-mobile-p-68129