It’s only a wee bit offroad(not great either, more intersting than the road though) and it will take you by the falls of clyde, and then onto some quieter roads. I don’t think there is a way to it totally off road. But this will keep you off the main road, adds more distance obviously. It’s also hillier this way, the road is an easy scoot along.
As for tinto hill itself, I’ve only done the main path which is great, I suspect the south path may be too steep, the path heading to the east looks doable though, as I say haven’t done either, just the up and down the main path a few times(bottom half is much better than the top.) So the south path may well be doable.