Any possibility of synchronization with SF Symbols similar to what Keyboard maestro did? I think devonthink’s support for icons is very poor, it could create some native feature of the software or include some standard icons similar to what Omnifocus does.
No, there are no current plans to give such access to icons. You can already change icons in the Info inspector or popover via copy and paste or even drag and dropping an image to the thumbnail.
You didn’t mention what you’re trying to do with icons or symbols? Context and intent matter when looking for help.
With the Apple SF Symbols app installed, and with the character viewer (^-⌘-␣) configured to add all available symbol sets, there is quickly at hand several thousand symbols to enjoy at a keystroke.
And, since many of these are available in KM macros, a macro could probably do most of what you want the developers to change in DEVONthink, with no delay in satisfaction. Self-help can be gratifying.