Tight budget doesn't mean tight on meaning - honestly, some of the most memorable celebrations happen when you have to get creative instead of just throwing money at it. The homemade dinner route is solid, but you could go even deeper: ask mutual friends or family to each send a short video or written message about a favorite memory with her, then compile them into something she can watch or read. it takes time, not cash, and people generally love being asked to participate in something like that.
For the day itself, think about experiences rather than things. a long walk to a place she loves, a picnic with snacks you've put together, a movie marathon of films that matter to you both - these stick with people way longer than a wrapped present would. If she's into any hobby, you could offer to spend a whole day doing that together without the usual interruptions. You could also make something small but personal: a handwritten list of 30 reasons you appreciate her, a photo collage from your years together, or even just a really thoughtful card where you actually put thought into what you write instead of generic wishes.
The key is showing up fully and making her feel seen. That costs nothing and matters infinitely more than whatever's in your bank account right now.