This is not a bad idea, but out of curiosity, is this actually a serious problem on your network? The repo updates add no noticeable delay for me.

A few seconds each time. And twice because of the additional repo configuration for pkgbase, I think. So it's noticeable when it's called over twenty times.

Actually, auto-install-packages already passes unrecognized flags like -U on to pkg install, so this would be redundant. The suggested changes to desktop-installer should work without this.

You must be far from the package servers. I assume you've tried various mirrors?