I got a nice deal on the x280 and am happy with it, was also looking at the various X1 carbon. Two criteria I had were I wanted USB-C charging (since I have those chargers around and they can handle these laptops) and a single battery (eg. the T470s I have from work is nice but it has two small capacity batteries that each cost the same to replace as the full size single ones in the carbon and x280). One thing to keep in mind is some of the earlier X1 carbon don’t support NVME SSD (I think it started with 5th gen?)
Edit: another thing to consider is soldered RAM. Part of why my x280 was cheap was it’s only 8gb and can’t be upgraded. Since you’re looking at lighter weight things and using FOSS (and perhaps open to tinkering with things like ZRAM) that might be a useful aspect to focus on because there is probably a glut of such machines given how memory inefficient things are lately with every trivial app running a whole browser engine. OTOH, depending how many tabs you tend to have open and how many electron apps you tend to keep floating around, 8gb might start to feel cramped. Especially if you think you might want some VMs around.
One thing that would be useful to understand is the distinction between CMR and SMR