It’s not anything to do with maturing imo, 27 is pretty mature and sex tends to be be put on the back burner vs. becoming spontaneously more passionate after time passes. Idk what your goals are as a couple but if either of you wants kids it tends to be especially true, at least for the first year.

It might just be incompatibility, your gf might not be into anything outside of the “vanilla” and thats ok but it is a difference that will weigh on both of you. Another thought I had while reading ur post is I’m almost wondering if because she is so inhibited during sex that she’s actually not orgasming as often as she says.

Most people will let go/feel more comfortable the more trust/intimacy is built and 5/6 years is hopefully enough time to at least build a solid baseline of that. Barring major trauma —which she should seek help for ASAP if it’s the case, your gf might be acting vanilla because she’s feeling vanilla about sex life in general. You ask about her fantasies/switching positions but what about things like foreplay? Seduction? Focusing on things like touch as the end goal instead of a means to sex? It might take more time but what you’re doing now isn’t working for either of you.

She might be extremely shy/uncomfortable with her own sexuality, so rather than directly asking try slower/more focused intimacy that is 100% solely about her receiving (not oral or anything else direct) before sex and gage her response w/out putting pressure on or making her feel like you’re expecting a specific response from her, I know that’s a lot and maybe I’m not making sense but lackluster sex can be the result of lackluster physical tension/build up beforehand that allows you to truly let go in the first place, especially when one partner is as anxious about it as she sounds