Definitely want to keep the Rich editor, we have that switched on.

Then, there isn't any easy solution we can provide you. 

Basically, users want to be able to bookmark a post for reading later - this might mean using an individual link to show one post, for example.

wpDiscuz doesn't have any relation to this. wpDiscuz is just a commenting plugin. You should find some other plugin for this purpose. 

This article should be helpful for you: https://www.wpbeginner.com/plugins/how-to-allow-users-to-add-favorite-posts-in-wordpress/ 

Basically, users want to be able to bookmark a post for reading later - this might mean using an individual link to show one post, for example.

 namely that when they're typing a long reply, they'd like to be ablt o resize the comments box

You can use the textarea, instead of quill editor. The option is located in the Dashboard > wpDiscuz > Settings > Comment Form Settings admin page. Just set "none" value for the "Load Rich Editor" option. 

More info here: https://wpdiscuz.com/docs/wpdiscuz-7/plugin-settings/comment-form/#load-rich-editor

Once you start to use the textarea, put the code below in the Top Admin Bar > Customize > Additional CSS > "Additional CSS" textarea, save it, delete all caches and check again.

#wpdcom .wpd-form .wpdiscuz-textarea-wrap.wpd-txt .wpd-textarea-wrap textarea{
  resize: ***vertical;***
}

You can change the red marked value if you want. More info here: https://www.w3schools.com/cssref/css3_pr_resize.asp

 they'd like to be able to 'bookmark' an individual post - perhaps a workaround would be being able to open it in a new window.

We don't follow you well. Please clarify a bit. 

Any workaround for these requests?

Love the update of the new version with comments added where the comments box is (because I moved mine!) so thanks for that because it's really made a huge difference.