A free and lightwave way to use an android as a control for a sbc?

[idkidk]

Something free, lightweight (and that works) that allows you to use an Android as a mouse/pad/keyboard? Possibly that they connect via Bluetooth.

I carry an SBC with me as a computer, the mini keyboard I had died, I can't wait 14 days for an AliExpress replacement, nor I want to spend €20 on fast delivered rubbish on Amazon

 

[LabRat 891]

I've used exactly what you're looking for, long ago (Galaxy S2 LTE). Too long ago to reference the app (it req. a 'server' on the PC, too)

This one appears to not require a separate server program but, I have no idea how 'bloated' the free app is.

Bluetooth Keyboard & Mouse - Apps on Google Play

Turn your Android device into a serverless Bluetooth Remote Keyboard & Mouse!

play.google.com

All of Appground IO's apps appear to serve your needs.

Android Apps by Appground IO on Google Play

Remote control for every device

play.google.com

The 'Pro' version might be worth it @ <$5, should the free version fit your needs [This is how I became acquainted w/ PDAnet+/FoxFi]