Hi Astroaurora.
The best way to find if you can do well with a 6’ or 7" thick mattress is to look at your BMI your most recent sleep history and any needs you might have regarding the sleep environment. You stated your weight and sleeping positions but not your height or BMI. Some specific reference points such as a mattress feel you have tried and liked would be good indicators too.
To help with your decisions there are some general guidelines here concerning height, weight, and body shape and some guidelines here about sleeping positions that can be a good starting point. The overview here and the more specific pages that follow will also give you some ideas of the effects of the different ways to put layers together.
Latex in general is a good/quality product. This said over time consumers discovered some issues to be cautious re Foam by Mail/ Foam Factory or any of their other websites (USA Fam/Foam Distributing) for some of the reasons mentioned in post #2 here and post #2 here and in this thread I’d keep in mind that their Dunlop page was not updated since 2014, also they are stating on their product page that they sell “100 percent natural Dunlop Latex Foam” Made in Europe" and I’d probably ask for some reliable confirmation of this. They offer a 10-Year Warranty and 30-Day 100% guarantee and if you decide to go this route I’d check their return policy and the “small print” on their customer service page for the details before purchasing anything. The key is to have a great deal of confidence that what you think you are buying is actually what you will receive and keep the written confirmation of anything you are told.
Let us know of your eventual decisions and additional questions as you move forward with your DIY.
Phoenix