That keyboard (even the "space saving" version) is huge and weighs a ton and doesn't blend with the idea of a portable system. If you want to use it at home and it's worth the expenditure to you, then fine. If I were tight on funds and wanted to homebrew a laptop I'd probably start with a cheap keyboard, with the idea of upgrading later.
I think nowadays, laptops don't need built in keyboards for the most part. It's ok to use a completely external keyboard. I've been using a Logitech K400 (wireless keyboard with USB dongle and built in trackpad) with my laptop ever since the laptop's own keyboard broke, and it's been fine. The K400 is a fairly crappy keyboard but it's cheap, and it's one of the few that I could still find with a built in trackpad. Those were once popular but now have gotten rare. I don't know why. Depending on your power and portability requirements, you might be better off departing quite a bit from the traditional laptop form factor.
I'm not convinced at all that an ARM processor will be as FOSS-friendly as an x86. For example, I believe the Raspberry Pi still depends on binary blobs. I have no idea about RISC-V but I think the available boards cost more and have lower performance than ARM boards or maybe even x86.
I've been content to run Debian on older Thinkpads. If I were really determined I'd use Coreboot (or whatever the current ideologically correct fork is called), which does work on some Thinkpads. But I haven't bothered.