Q: Strange download/upload speeds using the airport express as a repeater

Having the upload speed not change is odd indeed, but the loss of download speeds can almost invariably be attributed to the concrete walls in your condo. These do a poor job of allowing Wi-Fi signals to pass through them. Your actual Internet speeds are not affected by them, but the overall bandwidth speeds between the Internet server and your computer will be.

The best solution, albeit maybe not practical, would be to interconnect each of these base stations with an Ethernet connection and reconfigure the entire network for roaming.

Yes Powerline would be the other solution where running Ethernet is impractical. Most of the newest models should work fine for roaming. Remember with roaming you are not limited to Apple wireless routers, you could easily substitute dedicated Wireless Access Points for everyone except the "main" router.