I know you asked about thinkpads but I think you should consider a Latitude 7400. The non 2 in 1 model. It's got 1 m.2 slot, 2 Sodimm slots, and the back panel comes off in one piece and is easy to get at everything. The downside is the keyboard is hard to replace. It's one of those built in ones.

I run Debian on it and lock the CPU to 2.5Ghz and below and keep cooling pads in it and its passive. never runs the fan. Has amazing battery life and linux support. (shipped with ubuntu as an option). They're cheap too. The build quality is really good. It has the barrel charging port, usb c/thunderbolt port, hdmi, 2 USB A, headphone jack, micro usb reader, sim card slot, smart card reader for ports. Plus the trackpad actually has buttons which I like. I don't like the tapping that's become common now. The webcam has a physical shutter you can slide over it too. Overall a nice little laptop once you tweak it a bit.