Is a captured good tonearm cable better than a high quality one DIN connection?

you know the answer

NO ONE can tell you... you can only find out by yourself

What exactly have you purchased, and what exactly do you propose to do? it sounds like you have purchased a triplanar with a continuous cable from the cartridge pins all the way to the inputs of a phono stage. If that is the case, then no it would not make any sense to attach that cable to a box so you could then use your expensive interconnects into the phono stage. Anyway, so far as I know, the tri-planer is offered either with a continuous single cable such as I just described or a short external cable that terminates in a box with female RCA connectors on the other side. It is not conventionally offered with a DIN interface. But I suppose you can get it any way you want it.

Use the continuous.  Sell the $4k cable or use it for your second arm. You do have a second arm..?

A $4,000 tonearm cable? Damn, how much was the arm it was used with?!

I am certainly with lewm on this. On an arm like the Triplanar you do not mess with the cable. Direct is always the best and the only way I would ever order an arm. Some makers like Schroder will give you a choice of cables usually offering different metals, copper vs silver. They will usually now give you a choice of terminations as many of the best phono amps now use balanced connections. But, the cable is always a direct run to the cartridge clips.