Hi Schenkelini.
Welcome back to our Mattress Forum! 
It’s nice to hear that you and your wife are still very happy with your Flexus mattress… and congratulations on your new DIY Den twin mattress :lol:
Thanks for the update and for detailing your DIY bed and also for sharing your savings solutions. $463 is certainly a great price for a good quality 12" latex/poly combo. I’d be very curious to know how it works out for you and if you notice any difference in feel between the two types of latex. Your current mattress has a soft Talalay vs the NR Dunlop that you ordered from Sleep On Latex (one of our Trusted members here). Some people seem to prefer the feel of one over the other but of course “habit” also may have some say in this.
I’d keep in mind that Dunlop has a different “feel” and performance than Talalay and is less lively or springy. You can see a comparison between them in post #7 here but in the end only your own experience will tell which one you prefer. 100% natural Dunlop is a denser material and even in lower ILD’s it would be very durable. Because of the greater elasticity and performance of all natural Dunlop many people consider it to be better “value” than a synthetic or blended Dunlop although blended or synthetic Dunlop can also be a very good material that has many similarities to natural Dunlop and is also a very durable material. In terms of cost as you already noticed typically Talalay is a little pricier than Dunlop as it requires a different fabrication process which is more expensive.
I am looking forward to your updates once you receive all components and have a chance to test it.
Phoenix