Hi BS1515 and welcome to the Mattress Underground 
I’ve been mattress shopping for a month or so and have finally decided on a DIY. We’re in Canada so options are somewhat limited.
A DIY matress can be ultimately rewarding as it allows for ultra-customization of the layers to get the exact support and comfort you want…as long as you are willing to put in all the ‘leg work’ that may be needed for switching and exchanging layers to dial in the right feel.
I would keep in mind the basics. Any mattress build you and your husband finally choose will be unique to you, based on your Stats (height, bmi, sleeping position(s)and any underlying health concerns) - good job providing these! - and PPP (Posture & alignment, Pressure points, and personal preferences). You can also compare the Maatress Specifications You Need To Know to the Mattress Durability Guidelines to see how different materials hold up over time.
Current plan:
4" HD 30 ILD
2" HR 28 ILD
2" Dunlop Latex (use our current topper)
2" memory foam 12 ILD
I am wondering if the base layer at 30 ILD will be too soft and I should go with 40 ILD for better support and to hopefully help with the occasional back pain. Should we go with both the HD and HR foams for support or just one type and which one?
As you both have lower BMIs you have a lot of options in finding your ideal build. As you both are looking at ‘super soft’ comfort layers but are concerned about pressure point pain, keep in mind that using too thick of a comfort layer(s) may allow you to sink too far, bottoming out on the firmer support layer(s) below…which could cause the exact type of pressure point pain you want to avoid. Aside from that , you are correctly progressing from softer comfort layer(s) to firmer transition and support layers. You do want to make sure the memory foam is of a good density - we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support - but it looks promising. It’s good to have a firm base , but with for sleepers your size, it may be the ideal…you have your current Talalay topper along with a Dunlop latex comfort layer, so you can adjust how it feels for you both. Same for foam type, HD foam makes sense for your base, HR for transition, but you might want, as you mention, to use a firmer HD foam base, with a less firm HR foam above. If I’m understanding correctly, you want to use your talalay topper with a soft memory foam, and then a 2" Dunlop comfort layer? Bear in mind, generally you do not want a softer layer under a firmer layer as this can adversely affect the support ‘character’ of the mattress as a whole; you can read @Phoenix on layers here to see how comfort, transition and support layers interact in a mattress.
A few of the Trusted Members of the site are located in Canada; there’s MFC, carrying a variety off customizable organic latex as well as HD, HR and all types of foam for layers, full mattresses, and toppers; they can also assist you to find the correct components for the best sleep solution for your DIY; GhostBed Canada carries a few mattress models of similar build to your plan; also there’s Takasa for quality bedding and pillows, SnugSleep’s line of organic wool bedding, and Restmore Bedding as well.
We wish you great success in ‘dialing in’ your perfect mattress build! Possibly others in outer DIY community can chime in with their thoughts. Please drop by and let us know your impressions once you’ve spent some time with it.
~ Basilio