Hi DarkJojo,
Welcome to the Mattress Forum! 
While I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because 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”, firmness, or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable 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).
There are three different comfort designations for The Brooklyn Bedding Best Mattress Ever (Firm, Medium and Plush), and for side sleepers the Medium and the Plush are most commonly recommended. My best suggestion to you would be a detailed phone conversation with Brooklyn Bedding where you could discuss your needs and they could do their best to provide a suggestion that they feel would give you the best possible chance as success. As you may know, Brooklyn Bedding is a member here, which means I think highly of them, their products and the advice they provide.
I’m sorry your Casper didn’t work out well for you, but at least you had the foresight to choose a product that allowed for a return. I would make sure that you’re not using a true coil box spring under your new mattress, but instead something that is a flat and firm surface, more like a semi-flex grid network or a platform foundation. An old worn foundation that allows for sag or flex will make even a quality mattress sink and feel uncomfortable, and diminish comfort life while potentially voiding your mattress warranty.
Phoenix