As far as I know Z790 Apex will have same exact PCIe 5.0 support as Z690 Apex.
Which is no native 5.0 M.2 slots on the board,.. and 16 PCIe 5.0 lanes which are the X16 PCIe slots for GPUs.
If you run single GPU in top slot you will have X16 at either 5.0 or backwards compatible for 4.0, 3.0, 2.0,.. but still only X16.
Or if you run two GPUs and fill both slots in SLI you will have X8/X8.
Or if you run single GPU in top slot, and install a PCIe 5.0 riser card in lower slot you will have X8 on the GPU and X4 on the riser card.
The PCIe 5.0 slots become X8/X8 when both slots are used.
So basically your loosing, wasting X4 lanes if you use the Asus add-in 5.0 PCIe card for a single X4 M.2 SSD,.. as 4-lanes are lost, not being used.
They should have made that huge Asus add-in card support two 5.0 SSDs, or one,.. let the user decide as the slot supports X8 PCIe 5.0.
If they made the huge Asus add-in card support two SSDs in RAID-0 with 5.0 speeds, which would be smart, providing 70% faster transfer speeds than a single drive,.. and than your using all your 16 lanes with X8 on the GPU.
MSI is coming out with an adapter card for running two 5.0 M.2 SSDs in RAID-0 off the X8 PCIe 5.0 slot,.. will need to wait and see what shakes out when the card is released.
https://www.tomshardware.com/news/msi-gen-5-nvme-card-adapter-for-z690-alder-lakeI don't understand why Asus would not come out with a similar X8 M.2 card for the Z790 motherboards X8 PCIe slot like MSI did for Alder Lake,..
It's too early to speculate exactly what we are going to see with PCIe 5.0 M.2 support on unreleased motherboards yet.