Integrate the ones already in, so they become Germans first and Muslims (cafeteria Muslims, like the vast majority of Christians became) second. You can do that while the numbers are low, so that there aren't ghettos that run as parallel societies, mini Pakistans, where Muslim thought doesn't differ from their native countries. That way there is a much better chance of the West "corrupting" the religion with degeneracy like freedom of speech, freedom to offend your beliefs, freedom of women having agency, freedom of expressing your sexual orientation, etc.
For that sort of assimilation to work, the numbers coming in need to be severely curtailed, otherwise the rate of migration will win out over the rate of assimilation.
Also, limit this cultural relativism nonsense. Make it clear what Western values are and what norms are expected of the migrants to be followed (no matter if it conflicts with their religious beliefs). Look at Austria, seems like a decent first step towards assimilation.
https://www.nytimes.com/2015/04/08/opinion/austrias-islamic-reforms.html