Thank you all. Thanks GaëlG and codechefmarc for #6 and #8. I got far with these!
I needed to patch Plugin/Field/FieldWidget/MenuItemExtrasViewModeSelectorSelect.php to work with link view modes on node add and edit forms. The widget was picking up the node storage view modes rather than the menu_link_content view modes given to us by extras. This change seems to work in node forms and menu forms, hopefully others can confirm.
Sorry! I retraced my steps, and I can share a separate issue, which may not apply to your case. I should note that this patch doesn't solve a problem I have with compound class config lines - I will address these in another ticket if I get to it. In addition to the patch, I needed to separate my class toggles.
If I have one toggle for 'a.btn|Button' and a separate config line for 'a.btn--small|Button Added Styling Small' these work with the patch, using d9.5.11/ckeditor_link_styles v1.0.1. Even as a single toggle, I could not get a configuration with 'a.btn.btn--small' on a single line to be recognized after saving a node.
This patch shows changes required by the ck5 v29 1.0 Matcher pattern update
slattery → created an issue.
slattery → created an issue.