Congratulations on the new acquisition!
You're off to a good start with the 'gauge and some of the other mods you've mentioned. I support the earlier suggestion to look for mods that are easily reversible. That way, if there's any question in your mind, you can pull them off before taking the car in for service or whatever.
Before you get into fiberglassing a spare grill, you can test first if you'd like. Clear tape (packing, duct, etc.) will stick for a while (during dry weather, anyway!) and is a usable "stealth" approach to getting some data.
Do you need to carry more than two people, or would you rather have cargo space? I shaved off about 50 pounds by
replacing the rear seat with a load floor. Completely reversible.
No suggestions for convincing the wife; I'm on the back side of a failed marriage and clearly not a person to ask!
Since you're now driving a Versa, you might consider joining
Team Nissan.
Cheers,
Rick