system76 laptops?

Every Intel-based System76 laptop ships day one with Coreboot firmware preinstalled. The System76 firmware interface is also written in Rust using Redox libraries.

I think Raptor Computing Systems hardware supports coreboot. It's not Intel and not cheap, though.

Keep an eye out for system76 laptops on eBay. They are a bit pricier than equivalent generation ThinkPads, but they have supported Coreboot on their laptops for about three years now with continuous updates.

Be careful when searching; none of their AMD devices support Coreboot, and none of the desktops do, even if they are Intel. Also, if buying an older machine, make sure it came after 2020 or so. Galago Pro after galp6 and Darter Pro since the redesign are safe bets since they have Intel graphics. System76 has excellent documentation on their website for older laptops so you should find your answer there. These systems are designed in conjunction with Clevo, but one cannot simply flash Coreboot on the equivalent Clevo hardware since system76 has a different embedded controller.

Also, if not system76, the answer nobody wants to hear for Coreboot is, Chromebook.

MSI Z690-A sporting 12 gen would do.

While it doesn't satisfy the "under $400"-condition, I couldn't withhold myself from mentioning the likes of NovaCustom and Tuxedo. For the fact that both have been absent from the conversation while they otherwise satisfy the requirement of coreboot on a modern device. With the former, NovaCustom, being arguably the best vendor to buy 'privacy&security'-first devices with modern hardware from. Not for their entire line -mind you- but specifically their NV41 Series; which is -to my knowledge- the only Qubes-certified computer with modern specs and Heads. However (with Qubes and Heads pre-installed) it starts at $1224, which makes it considerably more expensive that your stated target (read: under $400).