How can I make a permanent change to a non-persistent live USB?

Solution 1:

Luckily length of new grub option matches length of existing options (it is not longer). I guess --- is used to inform end of options, I've just booted 20.04 LTS (I assume it either applies that same to 18.04 LTS or as one year passed it is Ok for you to use 20.04 LTS now) replacing "quiet splash ---" with "toram" and have not noticed any problems.

Therefore for that particular edit answers to that QA can be directly applied (editing on byte level of iso file or USB stick): How is it easier to make a persistent live drive with Ubuntu 19.10? See "Manual" section of sudodus answer or my answer with script based on sudodus answer.

$python
>>> len ("radeon.modeset=0")
16
>>> len ("quiet splash ") # not enough to replace only those
13
>>> len ("quiet splash ---") # just fits!
16

P.S. Assuming you do not need that splash image and don't mind output of Linux booting on the screen instead ;-)