I personally would recommend to go with the T440 series, as it's the last generation of Thinkpads where you can replace all hardware parts yourself. But it's core i-based, so it needs some work regarding Intel ME etc.
The Linux support is amazing, the only thing with Thinkpads is the synaptics driver which needs a config file to make it work as you want it to.
The thinkwiki contains a lot of information about all models, including a list of known problems. [1]
Before you install Linux on any laptop, I would recommend to check for BIOS compatibility. Sometimes there were OEM versions out there where the EFI vars could be deleted and then it's bricked unless you flash coreboot or reflash the original BIOS on it with an adapter.
If you want to go full tinfoil and libre, why not switch to a pinebook?
The Linux support is amazing, the only thing with Thinkpads is the synaptics driver which needs a config file to make it work as you want it to.
The thinkwiki contains a lot of information about all models, including a list of known problems. [1]
Before you install Linux on any laptop, I would recommend to check for BIOS compatibility. Sometimes there were OEM versions out there where the EFI vars could be deleted and then it's bricked unless you flash coreboot or reflash the original BIOS on it with an adapter.
If you want to go full tinfoil and libre, why not switch to a pinebook?
[1] https://www.thinkwiki.org/wiki/Category:Models