Hey CMSaiko, A Heartbeat is amazing entry! The gameplay is straight up solid, the movement feels smooth, the music is bouncy, it feels very fitting and sucks you in a flow similar to brotato or vampire survivors does. In order to underline that feeling faster and more meaningful progression would've been amazing! The only upgrade that felt worth taking for me was the additional hand, as the only other options were max health, current health or heal on demand with F although i didn't face any health issues. Upgrades like more projectiles while shooting, changing the projectiles flight patterns or make them chase the enemy, add explosions or any other twist that changes what you see on the screen.
I'd have loved an addition for auto attacking. Since there is no drawback on holding left mouse button and right mouse button is very situational, auto attacking should be an option so you just have to move the mouse around (or for more inclusiveness for handicapped, allow auto aiming at the closest enemy)
Another thing that could make the loop feel more rewarding is to let bigger enemies leave behind more XP. It feels "disappointing" when you fight that tanky enemy for 20 seconds only to get as much xp as you would for the easier ones.
The boss fight was very good! I loved the laser mechanic to dodge and it felt well balanced!
All in all very good job! I had fun playing it :)