An update, and likely my last.
After two months of trying to resolve this issue with Amazon, around 8+ missing / returned packages later, and close to 20 hours on chat, emails, and phone calls, I at long last have resolution. Over the course of those two months, numerous Amazon reps told me they were putting in a request for my packages to not be shipped with DPD. Yet DPD continued being the carrier. When I questioned this, numerous other reps told me it is not possible to outright stop a particular carrier being used for my shipments (at least, not for international shipments... it's apparently something they do more readily for US shipments, as there are so many carrier options).
Eventually, some weeks ago, after some ranting and low-level swearing (from me) a supervisor told me he was submitting a "deprioritization request" for DPD on my account. He said it might take 1 to 2 weeks to become effective. I said, "Isn't that what numerous other reps have said they submitted?" He assured me there was no record of anyone having lodged such a request for my account. He did it whilst I waited on the phone, and I received an email confirmation minutes later confirming what he'd done.
Two weeks later, I've placed a number of orders. There are now 8 packages on the way, and all have been shipped with DHL.
My tiresome dilemma with DPD seems to be over.
I suggest anyone else consistently having major issues with DPD make a similar request. I suspect you'll need to be talking with a Supervisor to make the request, as the common (phone/chat/email answering) Amazon reps are not in a position to make such a decision, and many don't even know it is an option. Keep in mind, I'd had close to 10 items returned to Amazon due to delivery issues. And a significant number of other items delayed long enough that Amazon's system figured they were missing in action (they eventually showed up). I suspect Amazon is not going to "deprioritize" DPD on your account unless they see there's significant (i.e., reduction of their losses) reason to do so.