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In the point-of-view culture in my story, all of the women in priestly families have two-syllable names beginning with vowels. (There are reasons for this, but they're completely tangential to my question.) I've gotten feedback from a beta reader that the character names look/sound too similar, even with my attempts to vary the specific vowels, the intermediary consonants, and terminal consonants if present. Examples: Elish, Ara, Efa, Eril, Aygo, Ina, Ilu. I'm guessing I should be using more "compound" sounds like "ch", "th", "br", etc.

I speculate that some phonemes are "more different" than others, and that I could make names more distinct from each other if I knew which those are. I also realize that some people "hear" names as they read and others don't, so it's possible that "hearers" perceive differences differently than "seers" do.

Within the constraints of the naming pattern in my world, how can I make characters' names look more distinct from each other?

You wish to maintain the brevity of the names, so I would suggest inserting silent consonants at the end of some, changing Ara to Aragh or Arah to differentiate it visually from the other names.

You could consider adding a z in the first syllable. Ara could become Azragh, Arzah or Arzagh.

Minor changes to names can be enough to help make them easier for the reader to recognize as unique to the character. Some of your names do sound similar, but they obey the constraints you imposed. Using consonants that are pronounced as vowels in other languages might help with the variety unless the constraint requires a true vowel to be the initial letter.

If it is the vowel sound that is what matters, in some languages H is not pronounced though it is present, with hotel being pronounced otel.

You might want to consider adding an apostrophe to a name, such as Eril and it could become Er’il.

Changing the appearance of the name can be enough to make the reader see them as very different names.

Using more compound sounds is a good start. Part of your problem comes from the syllables you're using - most of them are only 1 or 2 letters. The longest of your sample names is all of 5 letters - there's not a lot of room for variety.

I recommend adding more consonants to most your syllables, especially some of the less common ones (x, z, q, etc.). This way, your names will have different lengths as well as different spellings, which will make it easier to help tell them apart.

Your sample names could be changed to be something like this: Elish, Arax, Efa, Ermdril, Ayrgo, Ilthaz, Ilu.