Tell your partner they can't afford a new bike, but instead double the amount you would spend on yourself. However don't take that advice if you want to keep your partner.

All kidding aside you might want to look into used road bikes. I think they would provide better gearing, multiple hand positions, a more aero position, a relatively faster ride, and depending on the bike, attachments for racks and fenders. I would probably stay away from the more racing oriented bikes but rather look at cross bikes, sports touring bikes, and the like. You mentioned some pretty large cities in your original post, but i'm confused as to why you think they won't have a good selection?

For your current bike, new tires (slicks) will be around $20-30 a piece i suspect. A decent rear rack $25-45. Fenders $30 or so. Trekking bars about $30-40 if memory serves me correctly. Or perhaps you can get by with bar ends. These numbers are off the top of my head so if you look around you may get over or under that price, but i'm not sure by how much.