I came home yesterday near 9am. I messed around about 2 hours and I did Eureka for 4 hours. I got 38 Protean Crystal and 18 Anemos Crystals.
This is not an issue with Eureka, but with the way you engage with it. I now have 2 completed relics, and enough crystals to make a 3rd whenever I get feathers.

By the time I got to level 19 I had naturally earned enough crystals for 2 weapons and a full set of armor.

All I did was farm monsters anywhere from 3-5 levels higher than mine at whatever pace was comfortable, and ran to Notorious Monsters as they came up. No crazy tactics, no ridiculous time investment.
So if after 4 hours all you got was a handful of crystals, you're doing this very very wrong, and I'm confident that by the time you read this reply you will have figured this out for yourself, just by looking at how everyone else in the instance tackles the content. Unless you gave up.