with doas, you must allow TRUSTED_PKG_PATH to be passed through in /etc/doas.conf -- otherwise you set it in your user env, run doas, which then runs pkg_add without the variable, so it cannot find the packages you specified
you can check the mirrors. Every ca. 3 weeks there are new current-images. And you don't need to run fw_update each time, it works automaically by sysupgrade