Hi p-asadena,
Welcome to the Mattress Forum!.
I don’t know the specifics of your mattress but if you have an “all latex” mattress it’s possible that some of the deeper transition and support layers in your mattress may still be in good condition and it may be worth considering mattress surgery and reusing the deeper layers in your mattress and just adding a new comfort layer (possibly with a new cover). There is more about mattress surgery in post #2 here.
As you probably know flexus is a member of this site which means that I think very highly of them and that I believe they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. You are certainly heading in a good direction.
[quote]I’m 5’11", 155lbs, female, side sleeper, don’t move much. I’m heavy but also relatively lean and muscular - which means I’m pretty sensitive to pressure points at the hips. I like a firm mattress with a softer comfort layer - but no plush bed for me. I also sleep alone - most of the time 
I don’t really have a specific question, but I’d love to hear people’s experience with buying a new mattress for this specific issue of SI joint pain. What did you go for, what worked for you, what didn’t ? Especially regarding the layers ? Is there anything I should watch out for ?[/quote]
Hopefully some of the members here that have SI pain will see your post and share their comments but I would keep in mind that the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort” or PPP (Posture and alignment, Pressure relief, and your Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), any specific health conditions, or “theory at a distance” that can possibly be more accurate than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
In other words there isn’t a specific mattress that is “best for SI pain” because each person can be different and a mattress that would be a great choice for one person with SI pain in terms of PPP may be completely unsuitable for someone else with SI pain to sleep on because of differences in their body type, sleeping positions, or individual sensitivities and preferences. Just like clothes or shoes … a mattress that fits one person perfectly may not fit someone else at all.
While other people’s comments about the knowledge and service of a particular business can certainly be very helpful and a forum search on Flexus (you can just click the link) will bring up many more comments and feedback about them … I would be very cautious about about using other people’s experiences or reviews on a mattress (either positive or negative) as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you and in many if not most cases they can be more misleading than helpful because again … a mattress that would be a perfect choice for one person or even a larger group of people may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range or have similar health conditions) and reviews in general certainly won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress that would be “best for you” (see post #13 here).
The only thing that I would really trust when it comes to choosing a suitable mattress is your own careful testing and personal experience.
Phoenix