"I think i should just try TTA which is just a simple division of distance to go with the current speed"

NO!
ETA is simply Current Time + TTA
so everything I said about ETA in the real driving world still applies!

Sure, you can just divide distance to go by current speed - but the result is virtually meaningless in the real driving world: it assumes that the entire journey is completed at uniform speed. I don't konw about the roads where you are, but in the UK that is an entirely unfounded assumption! :-(

I suppose dividing distance to go by the current average speed would be a little more useful as an estimate of TTA, but it still can't account for the possibility that the roads for the remaining distance are much better or worse than those so far; ie whether the current average speed is likely to be maintained.
And, of course, the real-time traffic conditions...!

So from a driver's point of view, for a practical device, it wouldn't be very useful in real life. However, that doesn't necessarily invalidate it as a Project excercise