As I understand it, Microsoft Defender may not be open source, so you have to trust that what you're running really is Microsoft Defender and that it's behaving

As I understand it, Microsoft Defender may not be open source, so you have to trust that what you're running really is Microsoft Defender and that it's behaving correctly. Given that, a possible upside with ClamAV could be that you could verify the behaviour of the processes on the installed systems, to make sure you're up-to-date and have the correct software and signatures installed. That's not unreasonable - Microsoft's software delivery pipeline should be trustworthy and their security reputation could be damaged if an issue were discovered here.