Would this affect battery consumption though? The Wireguard protocol basically does not send/receive anything (when persistent keepalive is disabled, which it is by default) unless your phone actually wants to send something, and with Androids VPN API I think it mostly is as power efficient as it can be. As far as I understand having it in the Kernel only really provides performance (speed) gains, and maybe tighter integration into the overall Android ecosystem

Hmm, it might be on newer versions? Not sure, but in general it is build into the Linux kernel these days. I think the main problem is that most Android devices ship with outdated kernel versions.