The External ones should "just work" in almost all cases. an external USB device doesn't "have to" inject" any drivers to run. That is what the standard product is for. Same with DisplayPort. if implementing the protocol it should work. All the stuff with external edge ports should "just work". ( there are some funky USB drives that try inject drivers on first insertion to enable non USB features. Those are going to die ... and die hard. Loading drivers on the 'fly" just isn't going to work on Apple Silicon Macs across the whole range very well. And over time ( at least when get to stopping the Intel port, if not sooner. ) not at all. )Here is something I was just thinking of that I haven't seen mentioned in this entire thread. The entire purpose of the Mac Pro was that it can be customized to the users needs and desires,
The current Mac Pro has a couple of internal connectors. USB... see above not a problem. SATA. Again should be a problem for a pure SATA drive not deviating from the protoocls. PCI-e cards. Here may be getting into a bit of a slippery slope as cards sometimes have UEFI boot drivers so can do work before get to a full operating system. Technically, most of those are platform specific. ( either x86/ARM64/Power/etc specific. ) Pretty good chance that that will be a problem not specific to just Macs. ARM based laptops, servers, workstations from other vendors will stumble upon that isue also if just have x86 drivers built into card's firmware.
UEFI 2.5 has a bytecode (EBC) driver extension put in but probably not many folks following that.
It pretty much was an attempt to go back and cover what OpenFirmware had, but of course with a different implementation. Byte codes are a bit slower, but in the platform neutral boot manager phase, where just running driver through an interpreter because not trying to do whiz bang applications. Just trying to set some parameters and pick a disk image to proceed with.
USB , Thunderbolt , DisplayPort, HDMI , etc ... not really going to be a huge problem. ( Thunderbolt is on a slippery slope but if it is still bascially univeral across the whole Mac line up then there will be a 20+ Million per year market for those accessories just from Macs. Even if Windows on ARM 'fails' and only throws another 1-2 M on top that is still going to big enough for a viable ecosystem. )and that there will be an ecosystem of accessories for it.
I don't see that happening and I think the transition to ARM will ensure it doesn't happen for the Intel Mac Pro.
There is a potential problem in the discrete GPU space though. If Apple is out to remove them from a larger fraction of the Mac line up. So for example, MBP 16" and iMac 21-24" and the entry iMac 27" all loose them. That many also diminish some of the external PCI-e enclosure ( and eGPU) market. if it is just Mac Pro's that don't use MPX modules then the sales could get low enough to where get back to 2009-2017 era where there is little to no movement on "Mac Cards" in large part because market is too small. AMD and Intel fighting over a shrinking pool of discrete GPU opportunities may see one (or both) largely quit chasing Macs. ( Apple could counter that by throwing both some money to stay attentive until most of the transition is over. )
For compute targeted cards though the server Linux market probably does mean will get some either arm64 or EBC. At least more vendors who thought about doing a non x86 firmware. Similar for fast (not fastest but better than 10GbE) networking cards. ARM servers will present a bigger market Mac Pro may be able to bow-wave off of a bit. Add in Card SSDs... same thing.
There is a fair amount of work for the Add-in-card vendors to get done here. Probably better that they have have a two year lead time. Folks won't be able to "just ship the 3-8 year old" card and firmware as an option. But incrementally bigger firmware storage and a EBC option could get an "old, paid for' card going on the new systems when coupled to the macOS drivers that Apple is going to make just about all of the write anyway. ( IOKit is dying long live DriverKit. but that means everyone with a driver has work to do. ). The switch to ARM is going to kill off old , static products just like the 32-bit purge did. Probably bigger though for Mac Pro targeted cards.
If the Osborne effect sharply tanks Mac Pro sales and accessory makers largely don't get a return for doing MP 2019 products that could have a very chilling impact on the prospects for an arm64 based Mac Pro. But that is low sales as much as it is ARM. Thunderbolt on the arm64 other Macs could offset that a bit as there would be other PCI-e slots to fill there. (or PCI-e embedded like digital I/O box such as an video en/decode box for a laptop. )
if Apple and the accessory makers make good enough sales in the 2 year gap then it probably won't be as bad. By two years out reasonable card vendors should have some that x86/arm64 firmware or x86/EBC .
Similarly if the Windows 10 ARM desktop market makes any progress. ( not holding my breath on that one but it would have an impact here of cracking the "x86_64" only firmware mindset on a broader set of products. )
There is zero reason for Apple to drop out of the MPX module making business though. If all the other GPU A-i-C vendors flake, Apple will have to be the primary lead on solutions. I do not think though that Apple is going to try to push both AMD and Intel out of the Mac Pro MPX GPU chip business though. ( if they do a good job on following roadmaps and delivering top tier Metal code. )
P.S. Customizing the CPU package options 2-3 years after buying probably is going down. But the other parts ( RAM , storage ,etc ) are probably still going to be there.