Microshift SB-R472 7 speed STI can be found on aliexpress for less than $60 shipped. Have you measured the width of your rear dropouts or axle? A seven speed 14-28 freewheel would probably fit and they're less than $20 so there isn't a big investment risk. You'll have to put washers on both sides of the axle inside the threaded spacers to recenter the wheel. Alternately you could try to find a used short-freehub wheel at a bicycle co-op, then you'd also have a hollow axle and quick release. There are still a lot of old shimano 105 126mm axle wheels around. I would try that first because bolt-on wheels suck.