How arthropods could evolve a placoderm-like skeleton (semi-exoskeleton)?

Question

I recently made this question about the viability of a placoderm-like skeleton to permit arthropods grow till giantsizes, getting a mechanism which I decided to call "semi-exoskeleton" to due the presence of soft tissue parts over the exoskeleton to improve the articulations and letting the most parts of the body with the exoskeletal look.

That question was mainly focused about to solve the viability and biomechanical problems, asking about if it would work on land and if the mechanism are really useful or not.

So this question will be focused in how a current arthropod could develop that mechanism?

Having the previous images as references and considering some of the mentioned problems in the previous question like:

1. The biological mechanism by which these animals could grow within their armor without shedding as arthropods would, I have assumed that by a stacking of layers of bone dissolving into the lower ones as they grew. But I´m not sure how that worked and if this is possible for arthropods to use a similar mechanism.
2. The "semi" part, for much than placoderms could have an exoskeleton-like look, still having a layer of girded soft tissue over the bones plates in specific plates and a great part of its body made of soft tissue, appears the doubt about if arthropods could develop some "skin zones".

This would required to evolve a minimum of internal skeleton, spine maybe and soft tissue (skin and muscle) over specifical parts of the exoskeleton and maybe just girded in some other parts, how an arthropod could evolve something like this?

Answer 1

Double exoskeleton with the inner layer becoming what bone is to vertebrates.

During an arthropods moulting process there's actually a new layer of exoskeleton growing underneath, which inflates and hardens after the old top layer has been shed. This inner layer can be modified to stay inside the insect to serve as a makeshift skeleton. However if is chitinous it won't grow, so it may have to develop into something more bone-like that can be grown and corroded to grow along with the giant arthropod. Regardless this ingrown exoskeleton would only have three vertebra: head, thorax and abdomen. Actually the abdomen is made of multiple plates to keep them flexible, so it may be more than three.