This feels an awful lot like an XY problem, so forgive me for asking this, but for what purpose do you need the key repeat rate to be faster than the maximum?
Is it just a matter of wanting the speeds in macOS and Ubuntu to match up?
I'm just very used to the fast repeat rate on my Ubuntu workstation and recently started using my MacBook. I like having high repeat rate because it allows me to navigate much faster in my editor while developing stuff.
Can you learn to navigate by searching for the text you want to get to?[0] Or does your editor provide any facilities for jumping directly to the desired location?[1]
[0]: Beginner's Emacs tip. Should apply in other editors.
[1]: Example for Emacs: https://github.com/abo-abo/avy