For example, would it be legal to propose an amendment repealing the
bolded clause, have it ratified by 3/4 of the states and become part
of the Constitution, and then separately propose a new amendment
overhauling or abolishing the Senate that could be ratified and take
full effect with only 3/4 of states ratifying? Or could it even be
possible to do both in the same amendment?
The short answer is the no one knows because this has never been successfully attempted, so that the issue can be resolved in an authoritative manner. I personally, in my legal judgment, don't think that these approaches would be upheld (assuming, of course, that they could be passed).
It isn't even clear, however, if the courts would consider the issue of such an amendment's validity justiciable, in which case any objection to its validity might fall on deaf ears.
One could have unanimous consent from all states, or at least, from all small states impaired by the plan, which naively seems politically impossible. But so does every effort to expand the franchise (e.g. the passage of the 19th Amendment giving women the right to vote, which was adopted in 1920). Measures like these pass, however, when partisan majorities get control and care more about their partisan cause than their long term procedural interests (e.g. the interest of men in having a political monopoly) in a vacuum.
One way to get small states to agree to reform would be for a partisan majority wanting to end the status quo to create, or the credibly threaten to create, a large number of tiny states, gerrymandered to favor the ruling party's agenda, effectively stacking the Senate.
For example, suppose that Democrats controlled the House and Senate and Presidency as they do now, but had abolished the filibuster and had a safe majority for its legislation in both houses of Congress. The Democrats could credibly threaten to make the District of Columbia into not one, but forty new U.S. states, all safely Democratic leaning, in order to pressure small states into agreeing to a constitutional amendment that abridged their equal say in the U.S. Senate.
Still, the likelihood that a measure ending equal representation of small states wouldn't be opposed by at least some small states thwarting their purpose, seems far more likely in the current political climate.
The most plausible kind of reform that might be enacted without seeming like quite such a blatant case of blackmail and a power grab, would be to give every state equal representation in the Senate, but to change the power of the Senate collectively.
For example, a constitutional amendment might transfer responsibility for voting on Presidential appointees and treaties from the Senate to the House, or allow either the House or the Senate to unilaterally override a Presidential veto with a two-thirds vote, or might allow a two-thirds majority of the House to enact legislation which the Senate declined to approve within a particular time frame.