Upgrading beyond Mojave 10.14.6 on older Mac Pro 5,1 with Metal card and NVMe SSD in PCIe slot?

According to Mactracker, your Mac Pro 5,1 (2010-2012) can run no macOS version newer than 10.14.6 Mojave.

There is no sanctioned workaround for that.

That Mac is about sixteen (!) years old now and quite obsolete. You might consider retiring it and replacing it with the next Mac that will carry you another sixteen years. Any of the M-series Macs will run rings around that old machine.

As you can see from these benchmarks, all of the M4-series chips (plain, Pro, or Max) have similar single-core CPU performance – provided that you have enough RAM. Even a Mac with a plain M4 chip might have something like six times the single-core CPU performance of your Mac Pro (a desktop that was high-end for its day).

Apple has moved entirely to fast SSDs for internal startup drives, and all current Macs have USB-C ports which have support for USB 3.1 Gen 2, Thunderbolt, and USB4 20 Gbps / 40 Gbps.

Thus even a Mac with a "plain" Apple Silicon chip can deliver great performance for a lot of purposes provided that it has enough RAM and SSD space for the workloads you run on it.

With the higher-level chips, you get more powerful GPUs. You also get more CPU cores (especially "performance" cores), although that mainly matters if you are in the habit of running long batch jobs which can make good use of many CPU cores.

waiguoren wrote:

I'd like to know if there are any non-sanctioned methods

If there are, I do not believe that Apple would allow discussion of them here.

This Mac was never meant to run Mojave, but it does, and it does it quite well.

Never meant to run Mojave? Apple indicates that all Mac Pros with the model identifier MacPro5,1 are compatible with Mojave – once you install a Metal-capable graphics card.

Identify your Mac Pro model - Apple Support

waiguoren wrote:

Interesting; My last post with the link to the Macrumors thread was deleted... I guess Apple really doesn't want that stuff dicussed here! :-)

The post is still there – for now – although I would not rule out the possibility that Apple still might delete it.

Because there are so many people who access these forums, I'm sure that they are hosted not on a single server, but upon a whole network of servers, possibly "virtual" servers whose number varies with current load. (This is a very common approach for sites that serve many users.)

To reduce the amount of coordination work that these servers must do behind the scenes, they might be using a model where it is OK to show a "stale" version of a thread – especially to someone who is not logged in – or even to someone who is logged in but who is not a participant in the thread.

I've seen that recently with threads where I have replied. I'll go back to the thread, and recent replies – including my own – will be missing until I log in. Maybe the same thing happened to you.

waiguoren wrote:

[...] I also have several terabytes of storage on this Mac which I'd have to MacGyver a solution to access if I bought a new machine.

It shouldn't be difficult at all to pull your storage from that Mac and access it by means of external enclosure(s) when the time comes.