Hi jewels.
It seems that you’ve been on a little “mattress shopping” break and…congratulations on your weight gain! 
At your current weight, the “weak link” is related to your posture and alignment on a mattress construction using an 8" of 1.8 lbs. @ 20 ILD poly core which would not offer enough support to maintain your spine in neutral alignment. You can see some of the guidelines on PFA website ~ section 4.2.2. here. The ILD you mentioned is more appropriate for thin back pillows, tufting matrix, and very thick cushioning for seats. You’d need something over 30 to ensure proper support (You can read more about poly grades used for support here). While a 4-pound density memory foam would be within the recommended guidelines for a comfort layer for normal weight ranges I’d also keep in mind that a 12 ILD it would be quite plush and would give more with initial pressure compared with a higher density memory foam in the same ILD.
Phoenix