[Darryl Ramm]

Why do this when your Cheesegrater is upgradeable to run up to Mojave? Not too hard to do, and well documented all over the Web.

Last thing I would be going for is a Trashcan, especially if I had multiple PCIe cards. How are you connecting storage? Or can you fit what you need on the internal SSD (best option may be to max out that single internal NVMe SSD... which can get expensive).

You are planning on giving up the Omni and all DigiLink capability???... they need Pro Tools Ultimate (and a DigiLink License after 12.6.)

If you do want to stay on DigiLink, you need the HD/Ultimate license. As for the HD Native card... sell it and get a HD Native Thunderbolt. Way less faffing around, and quieter than an expansion chassis.

And you may be skipping over the most important thing, what exact errors are being caused by what exact plugins and how confident are you that an newer OS will indeed fix things? And what exact OS and Pro Tools versions do you need to get to? Given the spectacular improvements in Pro Tools 2019 I would be planning to get there. And given Mojave will work on an upgraded 2009, and adds NVMe SSD boot support I would be planning to be there.

If what you have works OK now I would be costing all options out carefully, you may not want to move just for some plugins, once you see all the cost.

Plan your upgrade in safer steps with full clean installs of new stuff, ideally on fast SSD, and ability to boot back to the old.