I've done my own map symbols since I bought my 5x, 2 years now, mainly to be able to see them on the watch with my 40+ eye's.
From time to time I find a symbol on the watch that I would like to change and often I then start a trial and error thing to find the right symbol id to change in typ file to be able to get the symbol in img file to change. Fun, but time-consuming.
Anyone here knows of a 'mapping table' between 'OSM symbol IDs' as I see them in a typ editor and 'Garmin IMG symbols'?