This is awesome for your first game, keep it up!
My feedback:
- Make sure it's possible to get some small upgrades in the upgrade tree in the beginning even if your first few runs are going badly. I had to play 3 or 4 games before I really got the strategy figured out and I couldn't really unlock anything in the upgrade tree to help me out. This could turn beginners away. Luckily, the fun of the game drew me in enough to keep playing.
- I would actually get rid of the timer, but that's more of a preference I guess. For me a strategy game should just give you the time to think about your choices, especially if you play it for the first time. Maybe reserve the timer for special levels or challenges? You as the developer play the game with lots of knowledge, while a new player will need to figure out stuff while on a clock, this is rather stressful.
- The combine mechanic is very cool with the different combinations you can make, however, it felt a bit clunky with the toggle button. Sometimes I would forget its on and get an upgrade pop-up and sometimes I would try to combine and not get the pop-up. Honestly, I don't have a better idea of how to implement it, but just putting it out there. Maybe it could already help if the characters have a minimum spacing from each other. Now I sometimes have to move some characters around to make them visible before I can start dragging to upgrade so I wouldn't upgrade the wrong units. This meant switch to dragging, drag all units to separate them and then switch to upgrade, then drag the unit to the unit I wanted to upgrade, then switch to dragging, then drag all units in combat position again.
Also, noticed you used the awesome unit assets from LyaSeek, which I have been using for a game as well.