Your monitor’s screen resolution will impact the available settings and external screen capabilities. Your 720p resolution monitor is very unlikely to be usable.
While undocumented, testing has demonstrated that monitors with a display resolution of 1280x1024 and below are only usable with iPad screen mirroring. As such, Stage Manager's extended desktop feature will not be available.
When used with a 16:9 widescreen monitor, having a native display resolution of 1920x1080 or greater, the full set of Display settings - and access to Stage Manager - become available.
The iPad User Guide lists the upper resolution limit as 6K - but the Guide offers no lower limit, or suggestion of aspect-ratio, being a factor for compatibility:
Connect iPad to a display with a cable - Apple Support
In conclusion, your monitor must be capable of 16:9 widescreen 1920x1080 (FHD) or greater to support Stage Manager extended desktop.