To solve the scale issue, you could just say that each card represents about 10 of that type of card, so every artillery card actually is supposed to be a squadron of 10 archers (except the shardbearer). That way it could fix the scale issue of each unit being the size of a chasm or the size of a mountain, and it would make sense as a shardbearer controls a lot more space then one unit. The only problem is that the movement of all cards is then exaggerated.
There is also something in the book that would be good to include. They mention that when there are uneven forces (120 soldiers vs 100 soldiers) the difference in soldiers creates a multiplicative difference, leading to the 120 soldiers crushing the smaller force. That would be cool to include, but I am unsure how this could be implemented.
I noticed that with the current rules, the infentry can go crazy. If you arrange your infantry in a square, then they can't be attacked except by a shardbearer and the artillery that has to be on high ground, which could be quite powerful
Also, when it says that artillery can attack in a straight line, I am guessing it can it shoot at something from an angle, such as 27 degrees and whatever. And can it shoot over enemys? How would determining what a creature that it can hit work?
Possible Solution: If it cant shoot over enemies or isnt on high ground, I think a line of sight system could be good if it is done well with eventually well defined rules instead of "eyeballing." Imperial Assult, a somewhat niche game, has an awesome line of sight system, but it is best on a grid. Essentially you draw an imaginary line from a corner of your card, and if it can draw two non-parellel lines to two of the enemies corners, it can target it. If the lines are overlapping another blocking thing (creature) it can't target it.