This is what your manual says about the expansion slots for the Taichi:
3 x PCI Express 3.0 x16 Slots (PCIE2 @ x16 mode; PCIE4 @ x16 mode; PCIE5 @ x0 mode) (PCIE2 @ x16 mode; PCIE4 @x8 mode; PCIE5 @ x8 mode)
* If you install CPU with 28 lanes, PCIE2/PCIE4/PCIE5 will run at x16/x0/x8 or x8/x8/x8.
* Supports NVMe SSD as boot disks
• 2 x PCI Express 2.0 x1 Slots English
• Supports AMD Quad CrossFireXTM, 3-Way CrossFireXTM and CrossFireXTM
• Supports NVIDIA® Quad SLITM, 3-Way SLITM and SLITM
• 1 x Vertical M.2 Socket (Key E), supports type 2230 WiFi/BT module
* The M.2 socket does not support SATA M.2 SSDs.
• 15μ Gold Contact in VGA PCIe Slot (PCIE2 and PCIE4)
(the presence of TM indicates that the Trademark symbol will not display in plain text)
and for storage:
• 1 x Ultra M.2 Socket (M2_2), support type 2230/2242/2260/2280/22110 M.2 SATA3 6.0 Gb/s module and M.2 PCI Express module up to Gen3 x4 (32 Gb/s)**
• 1 x Ultra M.2 Socket (M2_1), support type 2230/2242/2260/2280 M.2 SATA3 6.0 Gb/s module and M.2 PCI Express module up to Gen3 x4 (32 Gb/s)**
* If you install CPU with 28 lanes, the M2_1 only supports SATA type M.2 module.
Concerning the M.2 support:
The M.2, also known as the Next Generation Form Factor (NGFF), is a small size and versatile card edge connector that aims to replace mPCIe and mSATA. The Ultra M.2
Socket (M2) can accommodate either a M.2 SATA3 6.0 Gb/s module or a M.2 PCI Express module up to Gen3 x4 (32 Gb/s).
* The M2_1 connector is shared with the SSATA3_3; the M2_2 connector is shared with the SSATA3_2.
* If you install CPU with 28 lanes, the M2_1 only supports SATA type M.2 module.
And there is a module support list, which changes over time so you will need to check your M.2 purchase against the current posting.
Otherwise, I don't see an issue with using the M.2 with the graphics card of your choice, except that you will want a 40 lane CPU. The manual says the M.2 will share with the SATA port, but it does not say how it will impact the port. If it is a problem then you can always add a SATA controller card and go through one of the PCIe slots, but only if it isn't already being used, obviously.
It's a nice board and even has a TPM header, which is something I look for.
(edited for Trademarks)