What setup, if at all, would you upgrade to?
Having spent that much on an M1 Ultra (presumably for 'future proofing') I wouldn’t “upgrade” just yet unless it’s not doing something you need to do. Is it really slowing you down?
As others have said - it all depends on your personal workflow. The cheapest, base specification Mac can run Adobe Suite and Logic Pro and do useful work - but if you're compositing multi-layer high res images or need a 100-instrument digital orchestra loaded into RAM you may be looking at the max 512GB of the M3 ultra...
You can do some research with Activity Monitor and look at CPU/GPU usage and memory pressure while you work. If you are only really using < 10 cores then the faster single-core speed of the M4 Pro may well outweigh the M1 Ultra’s 20, slower cores. ‘Memory Used’ < 24GB would also be a sign of overkill, but even if it's higher you need to look to 'Memory
Pressure' to see if you're running out.
My guess is that a M4 Pro Mini
will be faster for your work than the M1 Ultra because, unless you're doing serious video editing, 3D rendering, AI training etc. that lights up all the CPU and GPU cores then the faster single core performance of the M4 will tend to win out - and the M4 Pro isn't exactly short of cores either. But, that's just a guess - there are other factors like memory bandwidth (the Ultra chips have insane memory bandwidth) which are hard to predict and/or maybe you're using Photoshop plug-ins that can really exploit all those cores on the Ultra.
Other things to consider:
The M4 Pro has Thunderbolt 5 - but how many devices that
actually use 80Gbps are you thinking of adding in the near future? Few offer more than a marginal improvement over TB4. Display-wise, TB5 is there for 8k and high-refresh-rate 5k/6k displays that don't exist yet. You don't
need TB5 for a 6k@60Hz display like the LG unless you're going to chain TB5 devices to it. Basically, TB5 is
good if you're buying new but TB4 on an existing machine is unlikely to be an issue for a couple of years yet and TB5 devices are backward compatible with TB4.
Your M1 Ultra has
six Thunderbolt 4 ports plus 2xUSB-A, 10GB Ethernet as standard and an SD card slot. The Mini only has 3xTB5, 2xUSB-C/3.1 and 1GB ethernet as standard. In many cases, being able to connect
more TB4 or USB 3 devices directly to the Studio is going to balance sharing TB5 via a hub (esp. for USB 3 devices)...
You don't have to pick up the Studio to turn it on.
Personally
(a) I'd stick with the Studio for a few more years - the only real reason to upgrade now is if you're worried about the second-hand value of the Studio falling in the future. Personally, though, I like to "overlap" my computers so I have a usable fallback machine.
(b) If I were looking at the top-end M4 Pro Minis + BTO extras I'd probably go the extra mile for the M4 Max Studio, because it's just nicer (unless you like form-over-function) and has better connectivity.
(c) I wouldn't get less than 512GB or more than 1GB internal SSD on a desktop Mac - that's more than enough to install a bunch of pro apps & library and keep your current work-in-progress. Any extra storage can go on externals. That's not just because of Apples insane storage prices - but if the Mac dies or goes of for repair then the internal storage is inaccessible, which really means you need an extra layer of backup.
(d) Don't get an Ultra again unless you've done the homework and established that your workflow can actually use all those cores.
