While you can successfully use an external display/portable monitor with your iPad in the manner that you describe, you'll not be able to directly interact with Apps that are displayed on the external monitor. At the very least, when using iPadOS Stage Manager feature, you will require a basic Mouse or an Apple Magic TrackPad. To fully interact with Apps when using Stage Manager and an external display, you will also require an external keyboard.
Unlike your iPad touch-screen, with which you can interact with the iPad, an external display/monitor does not respond to touch input. At this time, iPadOS does not support external touch-screen monitors. Similarly, Apple Pencil can only be used with the iPad screen.
The Apple Magic TrackPad works really well with a Stage Manager combo - and is available in black or white to suit your aesthetic preference:
Black: https://store.apple.com/xc/product/MXKA3Z/A
White: https://store.apple.com/xc/product/MXK93Z/A
The iPad User Guide documents the available TrackPad gestures:
Trackpad gestures for iPad - Apple Support
A basic three-button and scroll-wheel Bluetooth Mouse works well if you prefer a mouse. Don't bother with a sophisticated third-party mouse with multiple programable buttons as they are not fully supported by iPadOS.
For my own setup, I use the Apple Magic TrackPad with a Logitech K380 wireless keyboard (grey). Other Bluetooth keyboards that have the Apple-style secondary functions and command keys should be just as usable.