Hi Venture!
Welcome to our Mattress Forum! 
We are glad to have you with us on The Mattress Underground. You’ve definitely come to the right place!
If you are sinking more than before, this could be the reason for your lower back issues (sinking in too deeply and not providing adequate alignment). But this is a generalization and without seeing your product and how it performs I can’t be too specific in my conclusions.
In your case, the high 38 BMI presents special challenges and generally requires firmer materials (in the support layers especially). This could be firmer latex or innersprings (the type of support component would be a personal preference and in the right design either could be suitable) or even a zoned construction. The same overall guidelines apply with higher weights though that PPP (Posture and alignment, Pressure relief, and Personal preferences) along with using high-quality durable materials that will maintain their feel and performance for longer periods of time are the way to make the best choices. Heavier people, in general, will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal. I wouldn’t “rule out” any types of mattress and base your choices on your own personal testing. It is worth reading Post #3 has more information and suggestions about mattresses for heavier weights.
As a brief synopsis, higher BMI ranges will need more durable materials and components in a mattress and in a BMI range of 30 or higher I would include any 1.8 lb polyfoam or 4 lb memory foam as a “lower quality/density” material (relative to a higher BMI only) and minimize their use to a total of “about an inch or so or less” in the mattress. For polyurethane foam, if your mattress is one-sided then I would look for 2.0 lb per cubic foot density or higher. If the mattress is two-sided then I would use a minimum density of 1.8 lbs per cubic foot or higher. For memory foam (or gel memory foam), if your mattress is one-sided then I would make sure that any memory foam is at least 5 lb per cubic foot. If the mattress is two-sided then I would use a minimum density of 4 lbs per cubic foot.
Considering a hybrid foam over an all-foam mattress is a personal preference, but I would not take the foam or latex out of the equation only because you are not sure if you would have a reaction on a foam mattress for several reasons. 1 you may not actually be allergic to the mattress foams or latex (the products you are considering have foams) and 2. You may find that the feel of latex is something you like very much. I am not sure what you may have reacted to in the past with the pool floats but typically they are not made of the type of foams used in a mattress or latex rubber but are made either of vinyl or from closed cell foam, PVC (polyvinyl chloride) which It’s a form of plastic. Here is an excerpt from an interesting article from American Home Shield.
You were saying also that …
[quote]As for what I have found, I have narrowed it down, I think to one of the Helix Luxe, either midnight or dusk. I have also seen the Brooklyn Aurora mentioned a good deal.
I know, generally what they are made of, and would seem to be similar to hybrids I have tried in stores.[/quote]
I am not sure what you’ve read since you found our site but the two main areas of focus when purchasing a mattress are: 1. determining how long you will sleep well on your mattress and then 2. how well you sleep on your mattress. The first thing to focus upon is learning about mattress componentry and how to go about selecting a mattress. To do that, I recommend that you start by reading the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps, more importantly, know how and why to avoid the worst ones. I know that the COVID restrictions now make it difficult to go out and test mattresses, feel of materials and support levels you may need but I’d say that as with such an important purchase in terms of wellbeing I’d make sure that you take your time and do not leave stones unturned. When the companies do not list on the density or layer thickness, I’d first try to find this by email and chat directly from the company or try to find if other stores or consumers that have these specific mattresses have this detailed information listed. Once you determine the detailed specifications and check them against the mattress durability guidelines for a 30+ BMI, you can make a determination as to how long you will reasonably expect to sleep well on this mattress … or feel free to paste them and I or any other TMU expert or moderator will be happy to comment upon them.
Brooklyn Aurora has been previously discussed in Post #13 here that you may wish to look at.
specs for Helix Luxe mattressHere are some provided in May 2019 by another consumer. Also may wish to have a look at an older post of a high BMI consumer’s experience with Helix Luxe.
Generally, what" to choose in terms of either a mattress or the balance between comfort/pressure relief and support/alignment that may be suitable for a specific person is always up to the only person who can feel what they feel on a mattress and best left to personal testing and your own experiences or more detailed discussions with the more knowledgeable people at the retailers or manufacturers that sell a mattress you are considering. We are all very unique in what we each need and prefer. Some field testing would greatly simplify your search and help you make the best possible choices, but I’d make sure to find out exactly what is in the mattress, layering and densities in terms of on suitability, durability, and consider them against all the other parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
I’m looking forward to finding out where your mattress journey takes you … and of course, any additional comments or questions you may have along the way that I or any of the Expert members of the site can help with.
Phoenix