Have netbsd 9.3-STABLE running on a raspberry pi 3b with a 64-bit kernel. Installing it was as easy as burning the sd card
If you have packages to install properly, use this as your /etc/pkg_install.conf: PKG_PATH=http://cdn.NetBSD.org/pub/pkgsrc/packages/NetBSD/aarch64/9.3/All/
mastodon formatting might make that challenging, my apologies. you'll want the http and the full path. (you'll figure it out, I trust you)