This was really enjoyable, fast paced and shooting felt satisfyingly great. I think this was due to the fast fire rate and enemies quickly giving feedback on being hit.
If I could have one thing added other than more content, it would be a toggle for run. I always want to be running!
I managed to get fairly competent at shooting the doors from a distance then peeking diagonal into a room while shooting to avoid taking much damage. I think due to the current reaction time of the enemies and damage dealt with no health pickups, charging into a room isn't viable.
I was excited to read at the end that you're intending to develop this further. I hope that is still the case!
All the best with your future games.