You could sandbox it into a work profile that doesn’t have access to your main profile. Storage is completely segregated, and the work profile can be easily disabled when you’re not using it.
The best solution is obviously to choose another platform and convince your girlfriend to use that, explaining how this little extra effort on her part to use another app goes a long way with you in terms of appreciation and understanding of a partner’s boundaries and comfort zone.
The good thing about getting one from the start is that you can set it up to your liking from the get-go and won’t have to do it later. You’ll also get used to using it daily and see how managing two devices works for you.