I agree that the heter to place heavy burdens etc, is s bedieved, because the rambam says so beferush. I do not accept the assertion that there’s anything remotely wrong with owning a human being, which is what rabbi kook and others attempt to say. Rabbi kook largely had a value system not based on what the torah values and devalues – he also thought it was “wrong” to eat meat, for instance. he and many other modern people let their own biases and preconceived notions dictate their morals and values, and are we to be surprised when that hefkerus leads to distortions in halacha?
What you described re; tavi and eliezer are exactly the archetype eved kanaani that the torah is talking about. It’s a way of elevating a goy to a level of demi-jewish, as it were. It’s an opportunity to reach levels that without it are impossible to achieve….they keep not 7, but hundreds of mitzvos. And that greatness is lost when we denigrate the mitzvah as “lo dibra” or that the Torah was only talking in “those days”.