It's not (entirely) an issue of years. It's also an issue of number of episodes.
Season 5 will end with the confrontation between Fingolfin and Morgoth after the Dagor Bragollach. Beor, Haleth, and Andreth will exist throughout the season, but not spanning it. Their stories don't overlap. So, they are perforce each going to appear in a few episodes....and that's it. We only have 13 episodes to work with - and Andreth may be alive for a span of time, but she is unlikely to appear in more than 2-3 of those episodes, and will either be recast or have a ton of makeup on later.
We will show the romance of Aegnor and Andreth. But...that will be one episode. And we will show old lady Andreth later. We may add to her story, but I wouldn't count on that at this stage.