I looked into that category (of small and lightweight laptops, for travel) earlier this year, without the coreboot requirement. I ended up with a Panasonic Let's Note SZ6-CF. Also cheap - imported from Japan via eBay - I think it is better than the X200 series in almost every way, newer, faster, lighter. It might also have a better display than the default of the thinkpads. Only drawback: soldered memory (a crime against the longevity of those machines).

If you're interested in something of an even higher degree of robustness and are fine with an ARM device, check out the MNT Reform Next: https://www.crowdsupply.com/mnt/mnt-reform-next

I really wish we could get an MNT device with upstream support, if not an x86 processor. Having used the Pocket Reform, I think about it quite often. It's almost perfect.... but the ARM chip and all the warts that come with SoC crap basically is the one single thing that keeps me from using one.

Can't recommend starlabs enough, fully replaceable everything, coreboot, modern specs, Linux compat, firmware over lvfs