On sale is this Intel NUC extreme barebones mini PC that's a bit larger than your typical Intel NUC and not a bad option if you're after something in this form factor. There's a PCIe slot in this one that can fit small video cards like the RTX 3060 Mini and it has the Intel i7-9750 CPU that's still very capable despite its age.
Like other NUCs you need to supply your own storage and RAM. There's dual channel DDR4 SODIMMs, 2x M.2 NVMe/SATA SSD slots, 2x Thunderbolt 3 ports, HDMI 2.0a, Ethernet and multiple USB ports including USB 3.1 Gen2.
I wanted it so much two years ago, but now 9th gen h series cpu is so lacking the performance compared to current 12th gen, and the upgradability is a joke since you have to replace the whole compute unit which is even more expensive than the whole NUC. IMO better to get a small itx case and build your own 12th gen system.