Yes, once purchased, the Founder’s Pack Edition cosmetic items will be available in-game during Early Access and remain accessible so long as they are claimed before December 5, 2023. Please note, items currently available in the Founder’s Packs will not be available for purchase once the game leaves Early Access.
The game (and the Founder’s Pack Edition cosmetic items) is cross-save compatible on all platforms except for PlayStation consoles, meaning that you will be able to access your purchased content on all platforms except PlayStation consoles.