You don’t need a HighPoint SSD7101A, you need a single 1TB or larger NVMe SSD and a cheap PCIe adapter. While a bootable APFS RAID 0 is possible, it’s not very stable as every update will fail with “Failed to bless boot volume” because the update installer does not know how to write the boot volume to a RAID 0 volume. If you plan on staying on Mojave forever maybe that doesn’t matter to you. It’s a bit tricky to setup a bootable APFS RAID 0 but it can be done. The easiest way is to clone an existing drive to the RAID volume. I suggest getting a single disk, if you insist on getting an SSD RAID then you’ll be wasting your money, but that’s your choice. If you are running on a spinning rust hard drive (or 4) now, then getting an SSD will be the single best upgrade you can do.
You really didn’t need to do the Pixlas mod for a Radeon RX 560 as it only needs a single 6-pin power connector at the most. But since you already did, I would suggest upgrading to a used Radeon VII if you can find one as it is the fastest GPU available on Mojave (newer cards require Catalina for driver support.) The GPU will help to render faster, especially using FCPX. I would upgrade to an SSD before I would update the GPU.
As far as upgrading the CPU to a dual X5690 goes, well it would definitely help you because a lot of video editing processing is CPU based and doesn’t use the GPU and multi-core, although this is more true for Adobe Premier Pro than FCPX. However, finding a dual CPU tray is so prohibitably expensive that it would be cheaper to buy a whole new Mac Pro than it would be to buy just the tray. The single X5690 CPU you have is probably fine, and I wouldn’t worry about getting a dual tray at all. Remember that the second package (Intel terminology for CPU) will only help with multi core tasks, which while video encoding is one of those tasks, there are inevitably single core filters and effects that will receive no benefit from the second package. So while a second package would help with certain tasks like exporting, other tasks like dragging along the timeline would receive no benefit.