We run the same dungeons for 4 months, not half a year, then a new one gets added. There are other options such as hunts or other roulettes.

We reuse older areas for relic weapon grinds, quests and gathering. The older hunts still give endgame tomestones (currently aphorism).

Older dungeons and raids still give endgame tomestones through roulettes and they are reused for relic weapon grinds and moogle tomestone events.

(All of this content is still relevant when you catch up on old content that you missed such as old sidequests and old grinds)