Reviewer wants me to do massive amount of work, the result would be a different article. Should I tell that to the editor?
I've been R2 a few times: I consider the paper's quality clearly below the standards of the journal (because of lack of detail, for instance, to the point where I can't fully understand the paper's methodology; or lack of sufficient evaluation), so that in principle I should just reject the paper outright. On the other hand, there is something about the underlying idea that has promise, and I don't want the authors to get so discouraged that they bury it by resubmitting the paper to some third-tier venue. 

So I use Major Revisions as a kind of contract with the authors, laying out exactly the changes the authors would need to make in order for the paper to become acceptable, even if these changes are very extensive. The ball is now in their court: they can either take the time to make the required changes, with a high chance that the paper is then accepted, or, if they really don't want to spend the time, they can resubmit to a lower-tier conference or journal.

And of course, Major Revisions is also an easier recommendation for the editor to ignore than Reject, in case my views are not aligned with those of the other reviewers. In your case, it sounds like you only have two reviewers, who drastically disagree. You could submit only minor revisions, and hope that the editor agrees with R1 that the paper is acceptable without a lot of extra work. Or R1 might look at the revisions, and R2's review, and say to themselves, "oh, R2 is raising a lot of good points. They're right, this paper does need more detail to be understandable/reproducible!" The only way to know for sure is to submit a revision and see what happens, though one potentially useful piece of information: if the editor really did agree with R1 much more strongly than R2, they would have required minor instead of major revisions.

(PS: Writing up your research to a sufficient level of quality that it passes peer review does not "take away from your PhD research." It is your PhD research.)