AA but having a swappable battery tray

Microsoft did something like this with xbox controllers. There are additional contact points inside the battery chamber for a li-ion pack, so you could use a pair of AAs or their rechargeable pack that just fits into the same space.

I disagree about the batteries. Give me replaceable AA cells any day over a built-in Li-ion. Rechargeable AAs are readily available and quickly swappable if you keep hot spares. Much better option for long term serviceability.

Ah, if you need to build a .NET project that makes sense

Nuget is a the .NET package manager. Like npm or pip, but for .NET projects.

If you needed it for a published application that strikes me as fairly strange.