That's really cute. I personally won't use it (don't even use Fish), but wanted to give some tips. First, the code is intended to run each time a new terminal (more accurately a new instance of Fish is run) is opened up, right? If so, then I suggest to optimize a bit for performance, so opening a terminal stays snappy.

There are parts that do not change usually, so running and grabbing the content each time is a bit of waste. I'll suggest creating a single cache file with all the content that stays the same. In example following lines

awk -F'"' '/PRETTY_NAME/ {print $2}' /etc/os-release
awk '/cpu cores/{c=1} c{ print $4; if (/cpu cores/) exit }' /proc/cpuinfo

need to run only once, if the cache file doesn't exist already. At start of script check if the cache file exists, and read the file once only. The file could be in /tmp/ directory, so it gets deleted with reboot and should run once and always up to date. There is also a file cat /proc/loadavg, which you could use instead using uptime | awk. So instead you could just have a more simple sed or grep command. Instead:

uptime | awk '{ print substr($(NF-2), 1, length($(NF-2)) - 1)"% Load" }'

do

grep -o '^[^ ]*' /proc/loadavg

Note my code examples are in Bash syntax, not sure if there are differences that matters here. I see you like awk a lot BTW.

Edit: Changed the line with uptime | awk from your code, as I copy pasted the wrong line by accident before.