It is easy to self-host the frontend but be aware you can customize the relay list already. When you login with your magic string your relays appear as an array inside a url parameter: &relays=["wss://relay1.com","wss://relay2.com","wss://relay3.com"]

You can modify those right in your url to use whatever relays you want, and if you do, it propagates to the url you share with people who want to access your store. (The "view store" button has your relays in its url.)