The really simple setup for a single user is just a SSH server with access to storage and the git command. Assuming your laptop and desktop have SSH access to server, you can just:
For something slightly higher-tech, I recommend going with Forgejo (the fork of Gitea). It is really easy to set up and low maintainance.
Avoid GitLab for small setups, it is fairly resource hungry.
Yep, thats how that works. Every* i2p node also acts as a relay, which not only helps the network, but also your anonymity, by drowning out your traffic. It however only does this inside the network, it doesnt work like an exit node.
By default, it does run a proxy, so that you can access i2p addresses using a browser set up to use it. It also lets you use the proxy to access the internet over i2p, but you have to choose an exit node manually (tho iirc there is one set up as default, which is fairly centralized, but still should be anonymous thanks for the rest of the network). A slight difference from Tor is also that these are protocol level proxies, so you will for example not be able to connect to a clearnet ssh server over these.