Had the same issue as you with the Romer Duo Plus.
My daughter used it from 1 to 2 and that was it... Very long upper body
(by the way, the rule is that the head can go over but max at eye level otherwise not protection with impact)
We chose to get a Recaro Young sport next, which grows with the child and is a group 1-2-3. She is now 3.5 and still using the harness bit as there are several pieces you can take out to make space. I suspect when she is about 4 we will start using seat belt.
Have got for her a booster for city trips which is fine, but would never have her in one for long trips, both for security and comfort.
If it is of any comfort, even having gone through this, I am still very happy we have the Romer seat because my daughter went into it young (about 10 months) since she had outgrown the Maxi Cosi and would not have put her so young in a 1-2-3 seat, I was happy to have the group 1 seat.
As for your hubby... Not sure what to suggest, as with all parenting things, you guys must decide