Trying to work out a timeline for the relative ages of the Finweans is a ridiculous headache, but it’s definitely possible to have large gaps in ages and still have Ambarussa of age by the time of the Darkening. Here are my personal headcanons:
(bolded are taken directly from the Annals of Aman, everything else is completely arbitrary)
1170: Death of Miriel [none of Tolkien’s timelines make any sense if you accept the Shibboleth of Feanor version where Miriel lives until Feanor is fully grown, so this is one case where I prefer Silm canon]*
1185: Marriage of Finwe and Indis
1190: Birth of Findis [in the Annals of Aman this is the birth of Fingolfin, but that appears to be a version of the legendarium where Findis doesn’t exist; otherwise there’s no plausible way to squeeze her in.]
1217: Birth of Lalwen [Lots of people seem to have them as twins, but twins are extraordinarily rare among the Eldar and Ambarussa are actually the only not-half-Elven twins we know of. So I imagine her as his beloved little sister.]
1219: Marriage of Feanor and Nerdanel [as soon as he’s of age, because he’s impulsive like that and because the Shibboleth says they wed “while still in early youth”].
1250: Feanor invents the tengwar
1255: Marriage of Fingolfin and Anaire
1280: Marriage of Finarfin and Earwen
1300: Birth of Turgon and of Finrod
1362: Birth of Aredhel and Galadriel
1386: Marriage of Angrod and Eldalote
1401: Marriage of Curufin
1410: Birth of Argon, when I believe in him which is not usually.
1425: Marriage of Turgon and Elenwe
1430: Birth of Celebrimbor
1450: Feanor makes the Silmarils
1490: Banishment of Feanor
1495: Darkening of Valinor
We know that Finarfin and Earwen had four children within 62 Years total, and within 82 Years of their marriage, so I think it’s safe to conclude that having children within 15 - 20 Years of each other was perfectly normal. In my headcanon, the Feanorians average 25 years apart until Ambarussa, who are much younger than Curufin.
But, like I said, this is all just guesswork.
* More about why I reject the version where Miriel lives until Feanor is fully grown: This pushes back the birth of all Finwe’s other children by 50 years at least. And since Fingolfin can’t be younger than Maedhros, it pushes back the birth of Feanor’s children by a similar amount. That means Angrod and Turgon and Curufin were all the Elvish equivalent of teen parents. It also make some of the friendships between cousins highly implausible, since Aredhel would be the human equivalent of a ten-year-old when relationships between Feanor and Fingolfin disintegrated.
