Hi rolandtorres,
Bony pelvic areas have less surface area but are still the heaviest part of the body so they may be even more likely to sink in deeper than someone who was “rounder” and had more surface area per lb of weight compressing the foam.
I can’t see you on the mattress or feel what you feel so I wouldn’t even speculate about what the issue may be and I need to leave that to your best judgement along with the insights that the links I provide may give you. It may even be a combination of both (which I would suspect) because your larger size may need both thicker/firmer comfort layers and firmer support layers because they often tend to go hand in hand.
Having a problem on one mattress doesn’t translate into having or not having the same problem on another mattress that has different layering and design. The goal is to sink in evenly in good alignment in all your sleeping positions (see this post which may help you visualize this and provide more information that may help you).
Yes … you are correct that extra thickness/softness would solve both issues. Venus 1" toppers used to be sold at Cymax in 1" layers (and I’m not sure if it was genuine) but they don’t seem to carry it any more. You can find a 1.5" layer here which is the thinnest I found in a quick search. Other 7 - 8 lb memory foams may also work for you if you can find them in a 1" layer.
I would suggest that another 8 lb memory foam would probably also work (assuming that is the best topper choice in your circumstances and only your own experience can know this for sure) but you would need to talk to any suppliers that sold it to see how it compared in terms of its properties. There are also many 4 lb memory foam toppers that feel softer than most 5 lb memory foam and this may also be an issue and with your lighter weight durability of a 4 lb layer probably wouldn’t be an issue but once again you would need to talk with each of the suppliers to find out the specifics of the foam they offer. Different memory foams have a huge variety of different properties. The worst case is that you can experiment with memory foam toppers sold at the big box stores or other suppliers that have a good refund policy which can reduce or eliminate the risk of experimenting.
There really is no formula for me to know how you will experience any layering combination and my role is to help you understand the many influences and risks involved so you can decide on what may be the best choice for you and on your odds of success based on your own experience and insights in combination with conversations with better manufacturers and retailers that can talk to you on the phone.
It’s just not possible for me to make specific design suggestions in other words (outside of the guidelines I link such as the topper suggestions in post #2 here and the posts it links to) because it would take much longer exchanges or individual phone conversations and spending much more time with each member than my current 16 hours a day would possibly allow. My role is to provide more generic advice to help with how to choose rather than what to choose and to help connect you with people with the knowledge and experience that would be are able to help you with specifics that would apply to your particular circumstances (within the limitations of what they have available) rather than be the design consultant for members of the forum.
I wish that it was possible to do more for each forum member and give more specific advice but unfortunately it isn’t.
Phoenix