I wish that there were a simple answer to your conundrum over your wanting to ride with your wife, but your two divergent preferred riding styles (and your desire to gain more "endurance/stamina/speed") seem diametrically at odds. Every riding partner I've ridden with has has had some minor difference in riding technique or biking 'goals' from me, but your better half's unwillingness to get optimal leg extension is a real problem which can't be resolved by gears or position of the bottom bracket.
Perhaps you need to adopt two riding styles/bikes: a relaxed and pressure-free one when riding with your wife, and another one to embrace your own performance goals. It would mean that you need to get a better bike for yourself, and ride on your own to gain the latter, while keeping the former mindset fully accessible when riding with your loved one.
In any event, best of luck with your choice and whatever you do, keep riding!