My mattress has a crater now

So I’ve had my mattress for about 7~ years now. I loved it. It was perfect (sleepy's hush plush mattress) if it was still in stock I’d just buy that. About 2 years ago it started to sag in the middle and now I have a big crater I fill with pillows, but I can tell it’s starting to mess with my sleep.

I’m a side sleeper and weight 275lbs. I prefer plush mattresses or ones that feel softer. I’m starting grad school so my budget is tight (under $600). I’ve looking into some of the IKEA, costco, and Wayfair beds but I’m none of them seem similar to what I had.