I beat the game! Never used the bomber once. My final comps consisted of purely tanks mages and archers. With some clever targeting I was able to beat them. I mainly played the game with archers picking off the weak / far ones and mages and tanks killing near. It's up to you as the designer how you want to do army scaling, as at the end of the day it is your game! Maybe this problem can be remedied a bit by adding more classes if you plan on working on this game further
For the final level it is intended to get the bomber out and mages in that kill the weak. Level 17 is hard but winnable without the bomber but its a easy win with it. I don't managed to do level 18 without the bomber. Its great that you have found a way to do that! :) The original plan was to have more level with a higher army size at the end but i had not enough time to do this and so i had to reduce this.
I'm glad I managed to find an unintended strat that worked! For me, the bombers always just got into close range and got shredded before they could do anything because the enemies would target them first. All good that you couldn't add more levels for the game jam, since the game is very complex and in-depth already