Thank you,
And sorry for the very long delay in my reply. Last few weeks were tough on me so itch went into the back sit for a while.
I'm using Godot and I built my prototype using a serie of tutorial I've found on youtube (16BitDev is the name of the channel i think). I highly recommend it. Very beginner friendly.
I used several of his tutorials to learn about parallax, state machine, general shooting and spawn physics (in fact i studied his first 50 videos or so). I used some of his code, I modified other parts, added some signal management and sound management (but he might also have tutorials for these too by now).
You will find the foundations you need to replicate my prototype and even a lot more on his YouTube channel.
Thanks again !