Your best bet would be one of the Asus P8Z77 boards that have the thunderbolt header to allow connection to the Asus Thunderbolt PCIe expansion card if you can find either one still on the market. You can only get TB2 - TB3 did not come out until Skylake and by the Z77/1155 were obsolete.
Sound as if Win10 was not properly installed UEFI mode.
If Win10 installs UEFI mode there are 4 partitions
Partition 1 = Recovery
Partition 2 = EFI
Partition 3 = Microsoft Reserved (MSR)
Partition 4 = Primary Partition (C:\)
Do you have an EFI partition on the Win10 drive? Did you disconnect the Mac OS drive while installing Win10?
Does the icon in Clover have the Win10 drive labeled as Boot Windows EFI from EFI? If not, what is the label on the Win10 icon?
You cannot trash the EFI partition unless you reformat the drove or use a command line disk management tool to delete the partition. You can trash the EFI folder in the EFI partition if you want. Back it up first before you delete it. You would then need to reinstall Clover and the EFI Grub boot...
There is a listing on Dortania site.
Basically, any Keplar or Maxwell based 600/700/Titan GPUs or some of the Quadro K series cards for workstations.
You can get a 750TI off Amazon for under $200, but from your description of what you are using it for, the NVS 510 for just over $150 would do the job
Check your Mac OS drive EFI partition for a Linux folder. If you selected the EFI partition to install Grub UEFI mode then it usually installs in the active EFI partition, which would probably be the Mac OS drive ESP.
Since you have purchased all your parts and are now working on post installation issues you should start a thread in the High Sierra Desktop support forum.
3 icons are normal for just installed system.
Boot FireVault Prebooter from Preboot (to install FireeVault encryption)
Boot Prebooter from...
see Clover wiki on hiding boot entries
I have a vague memory of HP using the escape key on power up to select boot options instead of the function keys like most other OEM BIOSes.
Installing Linux UEFI you need to install EFI Grub boot loader (not the same as regular grub boot loader you may be...