freedom-u-sdk VS meta-raspberrypi

They are based on HiFive Unleashed OpenOCD configuration file on old Freedom U SDK but heavily modified for the latest version of RISC-V OpenOCD (and to enable debugging on U74 cores).

For this experiment distro that I ran benchmark on, I use the new 2021.05.00 image and replaced rootfs with my own. I guess as long as FSBL from fusdk 2021.05 is used, then the Unmatched will run at 1.2 GHz.

How do you figure Pis have bad integration with Yocto?

https://github.com/agherzan/meta-raspberrypi

For what it's worth, the entire Pi lineup is also well supported by Buildroot. In-tree, no less.

Embedded linux exactly. Major OEMs are using yocto. Check https://www.yoctoproject.org/

Been there. You don't want alpine or debian. Good gpos, but what you want is Yocto, which will let you build exactly and only what you need piece by piece including only the kernel modules for your hardware, the exact applications you use and no extras, and with a little extra tweaking, you can wire in Mender for ota updates and the ability to push custom images to clients that need specialization, or even fully unlocked images for customers that need it, plus if you're using an SD card, you can send users recovery drives instead of shipping full devices or let them build their own images without your proprietary code

I second Yocto. It's the kernel in use by the OpenBMC project

One last "option" is yocto but tis is not good for desktop, but it can be a fun project.