Magic Mouse dying symptoms?

In the past week, I've had problems getting my Magic Mouse (1) to connect to my Mac Pro. Once it's connected it works great, but it's a struggle to get that handshake at boot time or waking from sleep.

I'm trying to determine whether this is a normal symptom of a device dying after 10 years of service or if the problem is somewhere else in the setup. So here's a description of the set up:

• 2010 Mac Pro 5,1 with Bluetooth module disconnected from motherboard
• Broadcom USB dongle with Bluetooth 4.0
• Mouse is the only bluetooth device
• This setup has worked flawlessly for many years

Evolution of the symptoms:

• At first, I noticed it took longer for mouse to connect on wake (10-15 seconds instead of instantly)
• A few days ago, the mouse did not connect automatically. But using another device to go to the Bluetooth menu > Device > Mouse > Connect worked.
• Eventually that method failed and I had to reset bluetooth, forget all devices, and try different methods to eventually get the mouse to reconnect.
• At that time, I noticed the following error in Bluetooth Explorer: updateChainPower error: -3903. But not sure if it's related to the problem or if it's been there all along.

So I'm trying to identify the most likely culprit: the mouse, the dongle, or a software issue.