So I just went through this whole little journey trying to get P5800x working via M2 to U2 adapter, at PCIE Gen 4 x 4 speeds (albeit on a z590 motherboard and not a z690).
I tried 3 different m2-to-u2 adapter, two of which specifically stated Gen 4 compatible, but in reality a very loose definition of "compatiable" - and then the third card was the Startech PCIE Gen 3 card.
So techically the two cards that stated PCIE Gen4 compatibility did work at PCIE 4 speeds, however they threw thousands of PCIE Bus Errors, specifically WHEA-Logger Event 17 "Corrected Hardware Error". And aside from risk of data corruption, this also slightly reduced performance:
Errors after running at PCIE Gen 4:
Performance of P5800x on one of these cards:
All the adapters ran without error when the PCIE was manually set to PCIE Gen 3 in the bios (but obviously at Gen 3 speeds).
After a heap of research I came across an adapter solution from a company called Micro Sata Cables in USA, that includes a redriver in the adapter - which regenerates signals to boost the signal quality of high-speed interfaces. This ensured the PCIE connection met Gen4 requirements for signal loss and allowed the P5800x to power through at full PCIE Gen 4x4 and without any PCIE Bus Errors:
Here's what it looks like (adapter plus cable) and link below:M.2 M-key PCIe Gen4 with ReDriver to SlimSAS 4i (SFF-8654) AdapterSlim SAS SFF-8654 4i Straight to SFF-8639 U.2 Cable
*note: I got no association with this company whatsover, just relieved to get this working properly!