The Sonar Pen
We actually have been in contact with SonarPen's creator Elton Leung since 2018. We were hoping they can add Windows support ever since. To date, there's still no Windows driver but thanks to the support from SpaceDesk, we can now use it for Windows apps too.
The SpaceDesk app
2021.05.06 Update: you can use USB tethering for faster connection on SpaceDesk.
What can still be added: It'd be great if any of these programs can relay accelerometer/orientation sensor readings and act as a G-Sensor of Windows tablets so that we can directly control paper tilt in Expresii to direct the paint flow. Using other apps like Sensor stream IMU+GPS alongside is no good, as only one app can be in the foreground and nowadays smartphones would have background apps dormant after a few moments. We've tried putting the app in Battery Saving Exception or disable Battery Optimization (on Android 9) but to no vain.
In the above video, we also show the new features of Brush Tilt Control Pad and Auto Settle Timer: