Hi Frustrated Mattress Shopper,
The answer somewhat depends on what you want, and how well you can assemble a mattress that is suitable for yourself. If you’ve not already, you might want to read the introductory post, where Phoenix outlines how to find a mattress suitable for yourself.
Assuming its 6" of firm dunlop latex, it would be common to see anywhere between 3-6" of medium and soft latex on top. That said, given that it only had 1" of poly foam, it was likely either pretty firm to begin with, or, the Dunlop is soft enough to provide some comfort itself. In any event, your goal to reuse it would be to end up with a mattress that accomplishes the PPP outlined in the above post (posture, pressure relief, and preferences).
If you want a new mattress, with less experimentation with toppers, then shopping for a new mattress is a good alternative. A full talalay mattress certainly won’t be less expensive than a single layer in a topper. The risk of going with toppers is you are somewhat attempting to build your own mattress, and there’s likely going to be some trial and error involved (I.e. You can’t try it easily in a retail store first).
If you do decide to put a topper on it, you really just want to use just enough of a topper to accomplish the best feel. Using too thick and soft a topper can cause you to go from having too firm a mattress with pressure pain, to too soft a mattress causing your back to come out of alignment and causing pain from that.
… It’s really a preference thing as to how you’d like to proceed.
It’s noteworthy since you don’t have any talalay yet… Do you like the feel of talalay?
Also, If I recall, PLB sells some incredibly soft toppers, and so you do need to pay particular attention to not simply throwing the thickest softest topper you can find (e.g. 3" of 14 ild). While it might work out, it is more risky that it would be too deep/soft.