Actual bed problems. Need advice. 🛏

In the seven years me and my husband have lived together we have gone through four beds! He's a big guy, overweight (I'm guessing he's around 120 kg) and I'm a petite girl (45 kg) . The problem is that after a while the bed forms after his body, I guess, and it's like holes. Not actual holes but slopes kind of. I don't know the proper word, English isn't my first language. Do you get what I mean? It's so uncomfortable for me so when that happens we have had to get new beds, just getting new mattresses or matress pads is not enough because the "holes" are too deep. We could never really afford a expensive bed, so we've bought beds form Ikea or stores similar I quality and price. I know Ikea have 25 years guarantee on beds but I'm not sure this would be covered. We really need to get a new and a bigger bed now as our family is expanding, and I don't know what to get and how to avoid this from happening. It would suck getting a new expensive bed just for this to happen again. What should we do?