If it makes no sense economically there's a good chance (in general) that it was an intelligence agency of a superpower

Why do you think it's too costly? It's well known that there are divisions in DPRK that are focused on generating $ through crypto hacks and compromises.

We're dealing with reasonably sophisticated state actors here in this dark forest; not script kiddies.

Cheap $5 PCB + programmable small FPGA is a very viable route too for hardware implants; but even a microcontroller / ESP32/etc will probably do.

PCBWay will make anything you send them for reasonable pricing. Custom boards etc are not at all hard to manufacture, and for the amount of money you can get, of course you can team up with someone. I think the most people overestimate the cost of bespoke PCB creation.

Dangerous things sells implantable chips.

Eg. https://dangerousthings.com/products/apex-flex?Antenna=Narro...

If you slide through and ignore the writing, the linked implant teardown is wild - https://www.tibane.net/research/ledger-nano-x-implant

They did, in fact, appear to mass-manufacture a custom-built flat flex implant with cellular (LTE-M!), eSIM, and everything that hid right inside the Ledger, sniffing the SPI reads to the display to read the recovery phrases it pukes out and send 'em over cellular to the ringleader.

It's an EXTREMELY well built implant. I'm impressed. Multiple revisions, even.

The money's too hot to not build shit like this and slip it into the stream. Even a few thousand in R&D netting a six figure ROI is worth it, getting millions is just the icing.