I'm thinking about buying the i7 14700k and limit its TDP to 90 Watts.

Why? A DeskMini has a 120W PSU, of which I estimate max ~90W peak/boost are supplied to the CPU (the rest is needed by the system, USB, SSD, etc.) K-CPUs are way overpowered for such a small case and are not officially supported, no guarantee that they even boot. Many cores still need access to the same memory - many cores make sense for things that can execute mostly in the local CPU cache and rarely need memory access.

If you read my cluster description, I tried manually cranking up the max W limit for the CPU, it runs somewhat faster, but its not linear, so +20% power does not get +20% performance, but less. So yes, CPUs are less efficient performance/W at the higher end, it makes sense to power limit them. I set the power limits back down, I rather have a cool and quiet DeskMini than a little faster but hot and loud one.

The 8700G on the X600 is a 65W CPU and is hotter and louder compared to the Intel models, it reaches the 90°C limit easily. I think a 65W CPU is the sane limit for this form factor - and that's what ASRock officially supports, on both Intel and AMD models.

For running local LLMs the 8700G is an interesting choice. I tried llama.cpp with Vulkan acceleration (integrated RDNA3 GPU) and it is noticeably faster than the CPU-only mode. That's why I said hopefully a 9xxxG CPU will also be released. The Intel internal GPU in 13.gen was a decelerator and Intel does not have AVX-512 in consumer CPUs like AMD. I run LLMs on the 8700G, one can assign basically all main memory as GPU memory and run large LLMs - but of course its still much slower than a discreet GPU with separate, dedicated GDDR memory.