I added a 15pt weight -4 pt offset underline to this paragraph and I was wondering if it's possible to lower the opacity of the underline without changing the text opacity.
The Underline options won't allow you to change the opacity of the underline, but you can change the color and tint of it. So you can make it 50% black, but that won't make it 50% transparent. Except for white which does not allow tints.
The only way to achieve what you're after is to remove the underline and manually draw these as white boxes underneath the text, which you can then set to a lower opacity.