I've been looking at options for controlling my Ubuntu Media Center computer from Android. Mainly Mouse, but Keyboard is good too. Looked at a lot of stuff but the most interesting ones were:
remote-touchpad. A cool app that is server side only and uses the a browser on the Android side. The biggest issue is it is not that secure. Also only on Ubuntu as a snap and not on Debian.
Bluetooth Keyboard & Mouse (serverless). Fremium and not FOSS. Should have been good but I could never get it to work. The Linux Bluetooth system sees the packets, and the USB, Input, and XInput subsystems see the keyboard and mouse but no data flow. https://play.google.com/store/apps/details?id=io.appground.blek .
So in short I found KDE Connect was the most obvious but I really liked how XMouse worked in terms of ease of setup and simplicity. Frankly probably would have used it if it was in Play too.
I was thinking that BlueTooth would be better and more universal. The issue I found was FOSS apps seem to be missing and I could never get the one freemium app I found to actually work. This seems to be an area that needs a good FOSS alternative that actually works. Let me know if you have any idea of other options OR why I loose the data stream somewhere between the bluetooth stack and the Linux input subsystem.
Have you checked out Unified Remote? It's been the gold standard for Android-PC remote control for several years now, before KDE Connect was even a thing. It's open-source and is also on the Play Store.
As for your Bluetooth woes, I'm unsure, in my experience Bluetooth has always been a bit iffy, and it's succeptible to interference as it uses the 2.4GHz band. Better to stick to WiFi I reckon, it's faster and you don't get any interference issues (if you're using 5/6GHz bands).
Looks pretty good, but are you sure it's open source? I don't see the source linked anywhere on their website, and the "Description of Other Rights and Limitations" section of their terms of service suggests that it's proprietary.
Maybe I am mistaken but I do not think there is any server or client app code posted here. Looks like just plugins. Also saw no mention of licensing anywhere even for the plugins.