This is literally just a mockup 128x128 image. Not a single sprite pixel placed or line of code written yet.

Picotama 2 has the same rules as Onitama, explained for those unfamiliar:

At all times each player has two movement card options (red player's turn in image, choosing between upper right and middle left). The chosen card determines how the pieces (2 Masters, 8 Students, 1 Spirit) can move and always remain the same orientation (unlike the 180 rotation in regular Onitama). Here the red player has highlighted their Master and intends to use the top right card to move two spaces diagonally. Once a card is used, a timer of 6 turns is placed on it, all other timers go down by one, and the "1" card becomes available for the opponent. This makes red's choice an immediate game loser, as the next card available to blue is the lower middle, because...

There are two ways to win:
The Way of the Stone - Capture your opponent's Master.
The Way of the Stream - Move your Master onto the opponent's home square (where the Masters start).

The green piece in the centre is the neutral Spirit of Nature. Either player can choose to move the Spirit on their turn. Masters have full control of Spirits. If a Master moves onto the Spirit's square they swap places. A Spirit can not swap with a Master by moving onto the Master's square. Students have not yet learned control of Spirits. If a Spirit moves onto the Student's square they swap places. A Student can not swap with a Spirit by moving onto the Spirit's square.

I've not started yet coz I'm figuring out how the sprites/map is going to work. This will be my first Pico8 project.

I'm not familiar with this game (will definitely look it up), but I definitely hope you see the project through! Your mockup looks cool, and I would really love to see more board games on PICO-8.