An alternative is to use ARM-based devices: they typically lack microcode updates, and while TrustZone hypervisors are often present and the situation varies by device, at least some devices should allow the user to gain full control of the CPU without having to exploit anything. (I don't know how common that is, because every device manufacturer has their own boot process, and from some quick Google searching it seems like most of the time nobody bothers to figure out how the TrustZone stuff or the boot chain works, being satisfied with custom Android images...)

ARM Chromebooks in particular I think are theoretically supposed to have fully open source firmware, though the documentation is a little sparse... probably mostly for the same reason, lack of interest.

As far as I know anything Allwinner-based lets you run whatever you like with full control of the CPU, probably the same with other Chinese ARM SoCs but there seems to be more of an open source community around Allwinner for some reason. (The other main alternative is Rockchip.)

Old, outadated, with low specs only for those who need high performance. For 90% everyday tasks these laptops are more than enough. For me specifically and for many of my friends, they are 100% enough.