I’m currently on vacation, so I don’t have access to everything. The issue with cartridges is, that you either are in ROM (which voids self modifying code) and esp. with easyflash, you can only have an active bank at once. This requires a bit of setup, for easyflash you will have several banks that assemble to the same address (!pseudopc and !realpc come into play here)
You can mail me your modified snippet that shows the too many bytes error, so I can take a peek.