I have no idea if this is accurate or might suit you?
Points of Interest (POIs): You can upload and manage custom POI files in your library, and adjust their names and built-in icons through your profile, though custom POI symbols have limited visibility depending on your zoom level.
You can create a Custom Waypoint Symbols folder into which you can put all the icons you have designed- NB: There are specific requirements for them and their location.
They then need be manually copied across to your Garmin and ditto.***
Not all Garmin units can use then and there are still 'foibles' when you transfer files made on the go with these back to your PC.
***Sadly, yet another glitch which was never fixed by G. and ends up with all of your lovely icons being turned into blue flags on transfer from satnag to BC. (despite being identical and in the right places too
No app needed
No internet needed
No connection to Garmins' servers either
Bliss