Mavili, presuming shows the willingness for learning , so its a good thing to do...

as it is the phrase, the "Nice" part is unseperable part of it. We must use "Nice" for that phrase. But funny enough we can drop the "güzel´ bit from the phrase and it can still mean same.

Nice yıllara !

Nice mutlu yıllara ! ----- wish you [us] so many happy years

Nice sağlıklı yıllara ! ---- wish you [us] so many years with good health

Nice uzun yıllara ! ------ wish you [us] so many long years

 Here "Nice" means "lots of"

It is the route of "nicelik" = quantity (often coupled with nitelik = quality)

For example: Nice seneler (a birthday wish) literally means lots of years, and is best translated as many happy returns

They are supposed to be question words from Old Turkish:

Nice = quante (how much/how many in Italian from Latin) -> nicelik = quantity

Nite = quale (what/which in Italian from Latin) -> nitelik = quality

-lik usually = -ity