If you're looking to do a high-performance scratch-build in miniature, the ASRock DeskMini X600 seems like an intriguing foundation. Most mini-PCs come with a pre-soldered mobile APU running the show, but the DeskMini doesn't ship with a chip. Instead, you can install any desktop AM5 CPU across AMD's 7th and 8th-gen Ryzen range, provided your chosen one doesn't break the 65 W TDP mark.
You'll also need one with integrated graphics and, assuming you'll be using the X600 for some level of gaming...