Hello again 
I can already check 2940 tiles + spawn 2940 new tiles in one run without an error.
So I feel I am doing pretty OK already. Its close to half the map. (scientific journal here I come?! 
)
The spawning of the new tiles is just for me to visualize whats going on, so the yellow area is the ocean that is already scanned. I can also see this way that it seems to work great because it stops at land so at the end I will only have water that is actually ocean, then I can say that every water tile that is not ocean is actually lake.
Its similar to your logic just that I don’t check the material but work with my grid information.
But my question kinda remains, do you know If I can use the same code and just “run it on every tick until complete” to avoid the error? If so, what would be the blueprint equivalent nodes for such thing?
If something like that doesn’t exist I guess I will just split the whole map in a few chunks and then adjust my code for every chunk.