(I don't dabble in AI, but I have a couple of clusters running model rendering and CAM/CFD simulation jobs)
If current setup fulfils your need, no need for extra HW or peripheral for offloading. Without knowing your current setup, I think you will have to run against some issues before needing more processing power. Intel, AMD and Nvidia current gen can all process some sort of AI/HPC offloading. Once you do expect a bottleneck, you'll probably have a much better idea what process needs more offloading/acceleration. It really depends on your use case(s). You can then lookup what you need.
If you really want to buy something now and price is of no concern: You can't go wrong with CPU processing power (higher core/thread count), RAM and any current gen GPU. Intel Arc B-series, AMD RX 7000 and Nvidia RTX 4000 have acceleration and offloading capabilities (depending on your field or software requirements) and should be mentioned in any software usage guide lines. Nvidia has better support in general, so an RTX (or RTX Quadro) will always be useful. Usually more cores (TMU/ROP), more VRAM is better.
On the other hand: buying (expensive) equipment now, really doesn't save money in the future or even yield better results, if any. If you don't need it now, you won't need it in the near future, and on the horizon there may be better options available. It depends mostly on the software support and requirements, I guess.
Just beware: It (again) really makes no sense buying a RTX 3080 (for example) if you have no use for it now, because your application or software stack might not be optimal. Current lightweight applications rarely diverge into resource hogs...
Plex/ffmpeg seems a good example: Intel iGPU. up to current gen already outperforms any dedicated AMD/Nvidia GPU in realtime transcoding quality, and even when AI optimisation/processing comes into the picture, you can still offload the processing (via docker application container) to a remote processing node (wether CPU or GPU supported). I don't see a future path, where these applications by themselves would support/warrant realtime image enhancements via AI.
Or in the case of Immich: both CPU and GPU (Nvidia) load balancing is supported, you are most likely already to be using your current setup without bottlenecks. I find It unlikely that you would put a better CPU/GPU in your Immich server, just to "future proof" your current server machine. You are more likely to just put your current GPU in the server, and have it do it's thing in the background. You can then treat yourself to a shiny new GPU for gaming, and use that if you. need to help Immich a bit, but for the most part Immich can do it's thing on CPU just fine.
Maybe your Immich example is just the wrong example, but my advice, in case that was not expected 😉 is to just wait until you have a clear target. Machine learning is already ubiquitous, long before AI and machine learning became trendy to the general public. You are not missing out on anything, because you are already on the train and unless you are unhappy with your seat, there is no need to upgrade your seat ticket.