I had three fruit from my ~3yr limequat over the past few months all were low in juice, and somewhat tough compared to other kumquats and calomondins I've eaten - which are all good fresh. Is this normal for the variety? I wasn't sure how to tell when they ripen so I've eaten a all green one, all yellow, and in-between without much difference in taste.

I can't speak to the Eustis, but it's sister, Lakeland, is typically fairly juicy.

Both Eustis and Tavares Limequats that I've grown have somewhat less juice than calamondins but more than my Nagami kumquats. I have not had a problem with toughness. At my location they have to fully ripen for maximum juice yields. But with my Meyer lemons and Bearrs limes, they have plenty of juice without reaching maturity. I'm sure that sufficient sunlight, humidity and watering frequency are factors that determine fruit quality.

My other kumquats are identically cared for and have much juicier fruit that is less tough... this one has been somewhat unhealthy though so maybe that is a factor.

How can I tell when limequats are ripe? Once they turn yellow? Some time after?

I tend to pick them when there is a noticeable yellow blush to them (maybe half yellow). Perhaps, though, that's just because that's when I notice them. With limequats, and kumquats and their hybrids in general, I think you have significant latitude in timing your harvest (moreso than some mandarins, e.g.) - especially, in the case of limequats, if you are using them for juice. For eating, you might wait until they have fully turned.