I feel your pain Lorenzo, and many other designers do too, you're definitely not alone.

I have found the best way to deal with this is to let people know that if they want you to act like a developer, you're going to bill like a developer.

None of this lukewarm salary BS. No. They want a double or tripple-duty professional? They better be ready to pay a double or tripple-duty salary.

Value for value, it's only fair.

As a professional, you're selling companies future money at a discount through your experience and work that you do for them. The more value that you bring to the table, the more they need to be willing to invest to capture and retain that value.

Leverage is the name of the game here and I will also tell you that learning React and Flutter as a designer (while sucky) will take you places you never thought you could get to. Keep it up! :D