Hi marshmallow,
I’m sorry to hear about your experience … but I appreciate the update.
Depending on how you tested the mattress in a showroom (besides just testing for subjective “comort”) … a mattress can feel different when you sleep on it over the course of the night than it did in a showroom. Memory foam can also be affected by temperature and humidity so if the temperature in your room is different from the temperature in a showroom this can make a difference as well. Some of the other things that can affect how a memory foam mattress feels compared to a mattress in a showroom are listed in post #2 here. If you have gained 100 lbs then this would also have a significant effect on how a mattress feels and performs in terms of PPP as well and the quality and firmness of the materials that you would need in your mattress.
As you probably know, I wouldn’t consider any of the mattresses you you mentioned in your post (either the Serta, the Simmons, or the BIAB) because none of them use high enough quality materials that would be suitable for your weight. Even if they were “perfect” when they were new the risk that they would soften and break down much too quickly with your weight would be much too high.
If you are comfortable with an online purchase then there are many mattresses available to you that use much higher quality and more durable materials that would be a much more suitable choice. There would also be more suitable choices available to you locally that are also much less than 5k (or even half of this) as well.
Phoenix