Syncthing stores data like configuration, keys and the index database in it’s home folder, which defaults to $HOME/.config/syncthing.
Since space on your router seems to be severly limited, I would suggest moving the entire config directory to a different drive. You can do that by shutting down syncthing and manually copying the directory to it’s new place.
Afterwards, check how to change command line options on OpenWRT. You want to set the -home flag, where you can specify the new path to the folder.
I’ve no idea how the database grows, but I’ve been syncing around 200 Gigabytes with about 188k files in 20k folders for about a year and my database is currently 389 Megabytes big. Maybe resetting it from time to time will shrink the size down, I’ve added and removed folders a lot through time and I don’t know how good the database is with cleanups.
So if you want to play it save, have some gigabytes free for the database.
I remember @AudriusButkevicius mentioning some formula like “X bytes for every block times number of devices times something else”, but I can’t seem to find this quote.