After /r/TheDonald was banned, a bunch of right wingers did exactly this by creating TheDonald.win and other sites (including one for /r/TheRedPill), forming the win network. Honestly, the interface was better than Reddit and the site was very active, demonstrating that moving communities off-Reddit is very doable.
TheDonald.win went down because the admin saw the light after Jan 6 (https://www.washingtonpost.com/technology/2021/02/05/why-the...)