Hi cyrilp,
Eric Baird is exactly right. I've been building workstations for PhotoScan/Metashape for several years, and I often refer to the Puget System benchmarks by William George to look at how different CPUs and GPUs perform - he covers most of the options. I recently posted some info in
this thread that might be useful, but just to emphasize what Eric said - it really depends on your workflow and how you use Metashape. Some steps are CPU intensive and some are GPU-intensive. Some benefit from network processing (having multiple CPU/GPU nodes process chunks in parallel), and some need to run on a single machine no matter how many you have.
I have found that in general once you get up to ~6-8 cores, faster is better than more cores, but again, it depends on what steps. I am a fan of keeping my core count high while trying not to drop too much in CPU frequency, but some of that is because I have other pipelines that benefit from the high core count. William repeatedly says in the various benchmarks he has run that the core count benefit drops off after 6-8 cores too. So if you do a multiple-CPU setup keep that in mind and pick CPUs with a high speed clock. I was building multi-CPU machines earlier, but I've shifted to faster machines, while trying to maximize core count (currently our fastest single machine is a Threadripper 3960x with 2 RTX 2080 Super GPUs), and I'm moving into networking those machines now.
Personally I think that multiple networked fast machines with a few GPUs each are probably the best investment, and depending on your workflow, how much of it uses CPU-only steps, and how big you want to go, you might even want to have an overclocked Ryzen or two with no GPU or a lowly RTX 2070 Super, since you can fairly quickly eat up the price of a license with threadrippers and RTX 3080s. You say "without budget limit what would be the best upgrade?" ... well... a fast network of a lot of fast computers with GPUs and PCIe-connected NVMe storage I suppose...
Back here in budget-limited land, I'm frequently trying to balance performance gain with price increase - and the Puget Systems benchmarks are really good at helping me zero in on the sweet spot for price with GPUs and CPUs. For the work we do (Align/Dense Cloud/DEM/ortho) - the best performing machine is a Threadripper 3960x with 2 GPUs and 256GB RAM, but I'm about to throw in a third GPU.
For your machine, depending on the mobo and your workload, I'd look at adding another GPU before I do anything else if you're GPU-limited, otherwise, move your GPU over to an AMD 5000 series and put in some 3600MHz or faster RAM, or better yet, keep that one and build/buy another with a second Metashape license and network them!
Andy