Hey, I will drop my feedback below.
Depth vs. casual gameplay: I like it being more casual but you can maybe turn up the complexity in a single aspect like having a "resurrection area" around the cursor. Then you can have upgrades to that area (size, amount of revived units, maybe some damage, cc, etc).
Unit limit & performance: what about doing what those infinite runner mobiles games do? Maybe have an "unit sink" where you can spend some of your units for a pasive upgrade or a "miniboss" unit for instance.