I can recommend [Zephyr](https://github.com/zephyrproject-rtos/zephyr) for these things. Source: over 2 years ago

Zephyr, RIOT, NuttX are all open source RTOSes happy about contributions. Source: over 2 years ago

Microcontrollers are a good start. Boards are cheap and sensors are plentiful. You could get a yet unsupported board and port RIOT, Zephyr or NuttX. Source: over 2 years ago

Zephyr Project: Update to 2.5 or later.  Patches are available for prior supported versions. See the Zephyr security advisory for more information. Source: almost 3 years ago

BIOS replacement: Https://coreboot.org - T440p, X131e, X1C1, and xx30 series (no 61) or older Https://libreboot.org - similar to coreboot as of 20221214 release. Source: over 1 year ago

All of that code is also in the upstream coreboot.org sources, although these sources are less-well tested, generally speaking. Tested binaries (including a tested "more regular" payload like SeaBIOS or Tianocore instead of Chrome OS' "depthcharge" payload) based on that code can be obtained at https://mrchromebox.tech/. Source: over 2 years ago

Coreboot is great if you would like to enjoy working with the BIOS. https://coreboot.org/ r/coreboot. Source: over 2 years ago

Support guys like this: Https://system76.com/ & https://coreboot.org/ that do their best to remove everything proprietary at as low level as possible for Intel and AMD. You can also hack some older thinkpads that I think completely disable IME. Source: almost 3 years ago

If it is a mac, buy a similarly spec'd non-apple laptop, install coreboot, and then copy over your old drive's user data. https://coreboot.org. Source: about 3 years ago