Please check https://forum.syncthing.net/t/optimising-syncthing-for-low-end-hardware/14885.
I must say though that the memory gains may not be great. In my case, I have two devices with the same folders synced. One of them has been optimised for low RAM, while the other uses the default settings. The RAM usage difference between them is barely noticeable, i.e. maybe 5-10 MB at best (70-75 MB on Device A vs 80 MB on Device B).