Hello. Bootloader and baseband software are going to be closed source in your phone. There is no going around them. So you cannot have 100% of the rest of the phone as open source. Then there is also the code in TEE. That would also be closed source. I think if you dig a bit you'll find that the best you can do to run open source code is the kernel and major parts of ROM.
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You just have to blindly trust that this low level closed source software that you cannot replace would not do anything suspicious, and that it *cannot* be exploited either.

Hi. I know this is an old message I'm replying to, and you may have already left this issue behind. But I'm tempted to throw my 2 cents here. :)

if you can run VirtualBox (it is free to download/use and is FOSS -- I think) on your Windows 11 successfully, then install Mint or whichever Linux distro you'd prefer. It won't be as lightweight as WSL, but you can just plug-in Android device from there and do the adb or fastboot stuff, then close VirtualBox, and be done with it. No dual boot needed.