https://thunderstore.io/c/cities-skylines-ii/p/algernon/529TilesLite/
Enables access to all 529 map tiles, with a choice of one of three methods:
Unlock all tiles - automatically and immediately unlocks all tiles on the map when you start a new city (or load an existing one). Quite straightforward.
Provide extra tiles at start - adds 88 immediately-available tile unlocks when you first start a new city (all milestones will add their usual amount of tile unlocks on top of this); this enables flexibility with your initial start. Doesn't affect existing saves.
Allocate extra tiles to milestones - allocates the additional 88 tile unlocks across all regular game milestones, in proportion to the normal amount for each milestone. This enables steady unlocking over the course of a normal game. Doesn't affect existing saves.