I have a very nice existing mattress, queen size, but my partner isn't thrilled with how firm it is (I love it). Normally the solution as I understand it would be to get a mattress topper, but in this case I'd really only want it on half the mattress.

Now I can certainly buy a queen-sized topper and cut it down, or a twin XL (and also potentially cut it down), but that seems like a waste, and there's still the issue of keeping it in place; but I think maybe more importantly is that there would then be a 2+ inch difference in height between the different sides, which besides making rolling around more uncomfortable, I assume would also be a problem with our sheets.

Do they make split firmness toppers? Do they make like, firming toppers that I could place on my side? I don't want to have to cobble something together ideally, and I can't imagine someone hasn't come up for a solution to what I assume is a somewhat common problem. Should I really ditch the mattress and get two twin XLs (and upgrade to "King")?

We are looking at a king. The debate in my household is plush vs. firm. I have been reading about how you can make a firmer mattress softer with a topper. If we go with the firm, can i purchase a king size topper to make one side more “plush”? I suppose i could buy a twin topper, but do they make such a thing in a one-sided king, so that it would fit snuggly and I would not have to worry too much about wrinkles/bunching or shifting? Thanks in advance.