Maybe I missed it but you did not say what firmness, ILD numbers if you know them, are your 2" and 3" toppers, and I’m assuming this is a queen size bed.
My wife is about your weight and also has back problems, osteoporosis. An MRI showed she has several hair line vertabra fractures. We went with a 3-3" layers, soft, medium, firm combo from the Latex Mattress Factory, read our review of it here.
If your current toppers are soft and medium I would keep them and get another 3" firm topper. LMF also sells covers in various sizes.
And slats 8" apart on your foundation is, as you are probably already aware, way too much. They should be a maximum of 2.75" apart. Can you add more slats to your foundation? If yes, then you would not need a sheet of plywood on top of it.
Also quoting RF-Jens from one his posts . [quote=“RF-Jens, post:14, topic:41361”]
Nowadays, the DUNLOP process uses modern high-tech mixers that continuously produce an exact density with the same qualities and the same good and fine foam structure.
The statement “The TALALAY process enables lighter densities” is just as misleading. This is demonstrably false. With modern mixers, the DUNLOP process can produce foams with 55 grams per litre (dry) or less.
I bet that most of the so-called experts would not be able to tell the difference between a (modern) DUNLOP foam and a TALALAY foam (in a finished mattress) in a blind test.
[/quote] If you get another topper I would purchase a Dunlop latex one.
You also say that you have a herniated disc. I’m assuming you have seen a doctor, preferably a neurosurgeon, about it.
“Based on data reported by literature, it can be claimed that medium-firm mattresses offer more advantages to subjects with non-specific low back pain.”
Of course there are different types of back pain. What worked for my wife may not work for you.
We use Linen Spa mattress protectors. We have washed them many times and they still look new.; and will not change the feel of the mattress.