My point is that, among those scientific theories generally assumed to be fact, I think (Macro) evolution (i.e. one species gradually or in a punctuated fashion morphing into another species entirely) is one of the most poorly proven.

I don't think you have any grounds for thinking that. It's as proven as any other scientific theory ever is; we have huge amounts of fossil records illustrating long- term evolution of larger animals, and we can see evolution in real time in viruses and bacteria.

The main problem is the idea of 'species'. THERE IS NO SUCH THING AS A SPECIES in that sense. There IS a continuum of varieties, which extends over time and space; for convenience, taxonomists apply the word 'species' to variants which can be readily (if you are an expert) distinguished from other variants at the present time.

One common misconception is that evolution is something that 'happens'; suddenly, one species is transformed into another, the old ones all 'become extinct', and the new ones take their place. Nothing as dramatic as that ever happens- indeed, it can't.

Another is that there are objective tests of separateness, like chromosome count, interfertility etc. Viewing the world as populated by varieties helps to avoid this pitfall- you would expect some possibility of interbreeding between closely related variants, bigger problems between less related ones, and its absence when the separation is large. Which is what happens.

And it's no good arguing that this leaves us unable to distinguish between varieties. We can, just as we can distinguish between yellow and blue, even though there is no sharp dividing line anywhere in the light spectrum.