If the only need you need is sync repos without exclusions or transformations I wouldn't bother, it could work for you until it doesn't when they archive it or kill it like kaniko or so many other google products/tools.
Gitlab has really simple way to mirror from Gitlab to Github or other git vendors/servers
Not sure why a full desktop required, perhaps a Raspberry pi 5 16gb,
the pi 500+ or any other mini pc or a cheap chromebook would be enough to start, obviously depends on the kid's interests
it depends on your needs, but for me I set it up as the dhcp server and configure the router to go through the pihole. If you want to share it family and friends there is no better tool than tailscale, you can configure the pihole as an exit node.
it's a good post, however I agree with the comments there and here that a raspberry pi 5 with 8gb ram is an overkill for just running pihole. a good old Raspberry Pi 3 Model B with 1gb ram it's enough and it will still have capacity to run other things there. And of course pihole can run on an old laptop or desktop box you already have so no need to buy a device just for the sake of it. I would rather not run it as a docker container thou but that's just my preference
Gitlab has really simple way to mirror from Gitlab to Github or other git vendors/servers