Is the “40% Slaves” modifier from Slavers Guilds empire wide or per planet

So there are different methods of increasing relations with other empires:

• Establish an embassy: you can establish an embassy with the other empire in the same screen as the screen shot you provided. This will increase relations slowly until it reaches 100. You have a limit of 3 embassy by default but government type options can allow you extras.
• Rival the same empire: If the other empire has a rival, rivaling that empire will boost your relations by 25 ⁽¹⁾; this only lasts for the duration of the rivalry.
• Select xenophile traits: being a xenophile will increase other empire relations by 10, or by 20 if fanatic xenophile.

(1) Diplomacy - Stellaris Wiki: Opinion modifiers » Mutual rivals (17 May 2016)

Ethics divergence is either positive or negative.

If it is positive, it will push your pops' ethics further from your empire's. If it is negative, it will push their ethics toward your empire's. This has been demonstrated in another answer here and the Stellaris wiki.

So, if your empire is xenophobic, and these pops are fanatically xenophobic, then your empire is less xenophobic. Therefore, if you want them to become less xenophobic, you want to push their ethics toward yours. This would require a negative drift.

If you can't get negative, then at least less positive might work. It's not clear to me if the divergence applies equally to each pop, or if it might just be an average of a bell curve.

If you can get some of the pops down to xenophobic, then at some point they might end up less xenophobic than your empire. At this point, you would need a positive drift to continue to reduce their xenophobic tendencies and push them towards xenophilia.