imo, it depends wholly on dynamic and maturity level. There are plenty of couples with that age gap (as teenagers!) who meld together just fine and can be in healthy relationships. Can a 19 yr old take advantage of a 16 yr old? Absolutely. And the risk is higher the bigger the age gap is. But that's not a given, and the people involved are what's most important (How do I know? My partner and I were 18 and 15 respectively when getting together, and we've been together for nearly 15 years!).
You seem to know your characters, have considered the circumstances between them, and have said several times that it's unhealthy. As long as that's addressed in the narrative, you're good. And that goes for any toxic behavior: you can show it, use it a as a part of your story, etc. as long as you don't glorify it.
