Hiding payloads inside a mouse is child's play to a serious electronics enthusiast. There's lots of empty space in most mice, and there's 2.5 to 5 watts of power available from the existing USB cable (the amount of power the mouse needs is like a rounding error compared to that). Not to mention the USB data conection to the computer is already there, just waiting to be exploited.
You can do it with off-the-shelf parts and not even need to design and manufacture new technology. Look at this:
USB Mouse With Storage Added That was 14+ years ago and this guy was able to install a USB hub and flash drive inside a mouse. Nowadays stuff is even smaller. You could add a tiny computer with its own cellular data connection, using power from the existing USB cable, all with the resources available to a typical high school kid. The most challenging part would still be adding a camera with a useful image without the lens being terribly obvious to the user. That is the reason you won't find a spy camera in a mouse. There are just way too many easier things you could do with the space and power available.
But honestly this is 2025. Forget about the mouse. Advanced malicious devices can fit in the USB plug at the end of the cable.
O.MG Cable Anyone can just buy these things online. Think about that for a minute.
Also, the idea that antivirus software is going to block targeted malware is laughable. Every virus infection you or anyone else's computer ever got, failed to be blocked by antivirus software.