Doom on the 52840 is impressive!

I don't know if this is something I'd want to build, but I spent 5 years at an outfit that designed/built a variety of fitness devices like this, including some of the consoles on gym equipment you may have used.

You mentioned 'figuring out Bluetooth (BLE, please)" in another post. What are you having trouble with? The sample code in the Nordic SDK is pretty good in most cases.