Not much to comment on so far.
Took a bit to understand the piece limitation but I think the UI hints at it decently.
Between the cost of moving pawns and the fact that you have 8 pawns for 8 rows, the best strategy was just "1 pawn in each row".
I'm glad that you think the UI indirectly hints that there is a limit to what pieces you can place. I like to teach players without words whenever I can, so even though I have a tutorial, I didn't cover absolutely everything in the game.
I agree though that there is a stale meta I've learned through my own playtesting. The game is much more interesting in the early rounds, as you don't have enough mana and thus have to be smart on what pieces to place and move. However, once you get a full column of 8 pawns, you can decimate most waves with ease. I see two causes of this: