Well, on the first look it sounds like a solution is looking for a problem ...
But it's always good to have an ace in the sleeve.
I can't speak for MacPro's (and Your's 3,1), since I do only run intel-MBPs.
So far I've upgraded nearly all of my MBP (starting from the early2008 15/17"MBP4,1 up to the mid2012 non-retina 15" MBP9,1) to Mojave, because I wanted DarkTheme and of course the latest updates and App-support (which is now starting to fade away) AND keep 32bit-support.
On the unsupported c2duo MBP I keep Dosdude1-patched Mojave installed on GUID/HFS+ formatted partitions, to make them compliant to all earlier versions of OSX (including PPC).
Where necessary I have Dual/MultiBoot with (Patched)Mojave and earlier versions of OSX - or even later versions of macOS with OCLP/macOS (those one's on a GUID/APFS-formatted partition).
Important thing is, that HFS+ and APFS can coexist on the same harddrive,
but creating partitions(HFS+)/containers(APFS) requires a bit of thinking ahead, what space is required.
It took some time for me to sort out the differences between HFS+ and APFS and also the differences of (clone-) backup / restore procedures on both systems.
Partition (HFS+) is equal to Container (APFS). Size is static.
But within a Container(APFS) you may create multiple Volumes of dynamic size and you may install different bootable OCLP/macOS-versions onto different volumes.
On the MBP10,1 You have the options of (single-/multi-booting):
- Lion-ElCapitan (HFS+)
- 32-bit supporting Mojave (either the supported version on APFS, or the patched version with Dosdude1-Patcher on HFS+)
- OCLP/macOS (BigSur-Sonoma; on APFS)
So the space of Your MBP's SSD is the limiting factor, what to choose ...
You'll need at least two "static" partitions:
- one HFS+ for legacy OSX/Dosdude1-Mojave;
- one for APFS and it's "flexibly sized" Volumes for official Mojave/Catalina or patched OCLP/BigSur-Sonoma.
Same applies to Your MP3,1 , but You have the option of using different hard drives for different scenarios.
The MB3,1 is supported by
- officially: OSX Leopard/SL(Rosetta) to ElCapitan
- Dosdude1-Patchers: Sierra to Mojave(32bit) to Catalina (only 64bit) (either on HFS+ or APFS)
- OCLP/macOS: up to Sonoma (with optional
Metal-GPU for adequate performance)
(BTW: I'm writing this on a late2008 c2duo 2,93GHz MBP5,1 with OCLP/Monterey. No Metal-GPU. Apple-Maps and TV+ Apps don't work, since they require a Metal-GPU. The rest - even streaming video through Firefox - so far runs without major glitches. At least same situation like with Your MB3,1 but here on my MBP presumably much less horsepower)
So theese are the options.
If you encounter any limits with Your current soft- and hardware, that is related to the version of OSX/macOS, you'll have a bunch of options to upgrade the operating-system or hardware (and also the GPU on Your MP3,1)
If You have familiar workflows with your current (video-editing) software and encounter no limits, I'd stay with the current installation.
If You encounter limits, then try any of the upgrade-options, but leave your current working installation untouched.
If the limits a related to lack of performance (e.g. with 4k-video or higher), any macOS/software-upgrade won't solve the problem. The MBP10,1 sure does 4k well, the MP3,1 might do to with a Metal-GPU (but that's beyond my knowledge ...)
When it comes to it certainly the best option is to go Dual/Multi-Boot (on the MP3,1 on a separate drive) and try out.
