There is a way to remove everything but one app from android? I have an spare phone that I want to use with the music app 'Caustic 3'. My plan is to use this phone just to it. There is a way to remove everything and install just this program, like an ROM? Just turn the phone on and it open this app directly? And use all the machine resources to think just this? Thanks
Removing system apps can damage your phone and make it unable to function.  I would recommend installing an app like macrodroid then setting up a macro to kill off all the other applications and only run Caustic 3. If you really want to remove the other apps, then installing something like System app remover (ROOT) would do the trick. The first method is a lot safer however.