Great game! My suggestion is to add a Meta-upgrade using the Zodiacs/Planetarium items that you could bring into the run. The Zodiacs would be basic stat ups for towers and just increase viability at no downside, but the planetariums would either focus on specific synergy on a tower or give a niche mechanic.
Here are what I would put as such upgrades:
Aquarius: All Towers shoot faster/produce faster.
Aries: Spikes deal more damage.
Cancer: All Towers have more durability.
Capricorn: All Towers get a slight boost to all stats.
Gemini: All Charmed enemies have increased stats.
Leo: All Obstacles take more damage.
Libra: All charge times are slightly decreased.
Pisces: All enemies are slightly slower.
Sagittarius: All projectiles can pierce twice as many enemies than normal.
Scorpio: All Towers deal less damage, but deal more damage for each time they shoot.
Taurus: All Towers shoot slower, but shoot faster for each time they shoot.
Virgo: All Towers have less durability, but enter with a Shield.
These are just my opinions, and Im not too concerned with balancing, Its not like Im making the game. Even if they don't make it in, I still like the game for what it is.
Keep it going man!