It would be helpful if, for example, while setting up an Insert Text action, the Insert Token menu included, either within the existing sections or within a new section, an insert SF Symbol option.
Said option opens a picker for SF Symbols either as an image for the macro's Icon well, a symbol for the macro's name field, or the actual text to insert.
It would make sense to include a favorite section as the list is large, and likely only a small selection will be used.
I'm not really following you - do you want a way to insert SF Symbols into the Insert Text text field?
As far as I'm aware, SF Symbols are images, so that would be akin to adding images to the text field, which you can do for Insert Styled Text by Pasting, but wouldn't be possible for plain text.
You can insert Emoji via the system Emoji & Symbols picker (that's a different kid of Symbols than SF Symbols though). But they are text characters, so they can be entered in to the text field normally.
Using the word symbols is confusing in this instance. Within the SF Symbol app, either via the contextual menu or the edit menu, 'Copy Symbols' can be pasted into a text field and are therefore text characters while 'Copy Image' can be pasted into the image well, making them images.
Here are some examples I've used:
As palettes are not sizable (onscreen like a window is) and I put them in tight spaces to have them always on screen, I sometimes use palettes as text and combine text and 'SF Symbols" as macro labels and sometimes use palettes as images and combine an SF Image and some text (using Affinity Designer) to make a decipherable macro label. (Text characters can be symbols or alphanumeric characters, who comes up with this nomenclature?!?!?)
It takes some effort to make palettes that live on the screen with an app, and sometimes, for me, replace the app's toolbar with palettes of actions I want there instead of the choices Apple makes.
I doubt I'll ever learn to use the HTML option to create palettes as great as that control allows. This is perhaps a good thing as there are a few things I'd like to do before I die that don't have me sitting in front of a screen, and if I went down that rabbit hole, I might never reemerge. 
