what does" this far" mean? is it a synonym for"thus far"? and what does it mean in this example:" Oh I guess we made it, Or at least we made it this far, And it all looks smooth from here."?
This is actually a better question than I thought when I first looked at it. Especially for non-native speakers, I can understand how this far versus thus far can be confusing.
Far just means a long distance. That's the easy part.
Thus far means "to this extent or degree", which is the fourth definition given by Random House, BUT what I can add as a native speaker is that when used under this definition it retains an element of its primary definitions of "in this way/manner". Note that thus is mainly used as an adverb (and sometimes a sentence connector). definition here
This far means "to the extent or degree indicated", which is the ninth defintion given by Random House, and the only definition by them when this is used as an adverb, BUT what I can add as a native speaker is that when used under this definition it retains an element of its other definitions of "at present, near". definition here
Thus far is similar to so far or up until now, meaning it is used to indicate a period from some earlier point to the present point. For instance, suppose you and I have been collaborating on a research project for two months and I want to say that over that period I think we have made a lot of progress. I could say "Thus far our research has been very productive." The focus is evenly distributed over the whole period.
This far is more focused on the here and now than thus far. This far has some similar meaning to achievement or accomplishment in that it is understood there was a journey to get to the here and now but the focus is on the present. Suppose that you and I want to try out for our school soccer team and we know that it will be a lot of work, and especially the first two weeks which are heavy conditioning drills which we have heard are very strenuous. We join the team together but after a couple of days we are wondering if maybe we took on too much and even thinking about quitting. But after the two weeks we are but still alive and on the team and we know the practices will get easier going forward (though they will still be demanding). I could say, at that point, "well, we made it this far so we might as well stay on the team." The focus is on where we are now, which in comparison to where we were is expected to be much easier going forward.
This example I provided above is very similar to what is being described in the song lyrics you cite. The understood meaning is that they have overcome difficulties and that's a big accomplishment ("we made it, or at least we made it this far") and that in comparison to where they were, going forward from where they are now looks to be "smooth".