Try using rows instead of the JavaScript hack.

[FlipperPA]

Apparently, TextBlock can take rows as an attribute:

https://github.com/FlipperPA/wagtailcodeblock/blob/main/wagtailcodeblock/blocks.py#L52

https://stackoverflow.com/questions/69834824/is-there-a-way-to-increase-the-height-for-textblock-in-wagtail-content-panel

[lb-]

As per Slack chat, I think that in Wagtail 5.0 the built in text area widget should behave a lot nicer.
See wagtail/wagtail#10195