[Cthulhu_]

If you've already decided to move on (based on the comments here), you can go in with a last ditch effort; show them you were hired as senior, show your glowing reviews, and demand a promotion and/or raise.

If they refuse, that's cool, you give them your resignation - they probably anticipate a resignation already.

If they try to meet you partway, that's cool too, but it's basically them making up for something they should have done when they hired you.

If they agree to your demands, that's an excellent result. That gives you a starting salary to negotiate with if you then decide to move on anyway, but any immediate financial pressure should be off your shoulders then.