It does seem easier, yes. But there are issues with it, and we just have to deal with those issues as they come up. Because even if we avoid being explicit about a timeline, our storytelling will be implicit. If we change the flow of time or have timeskips in an episode...we do have to indicate that in some way.
The reality is that we are placing Turgon's dream from Ulmo after the Dagor Aglareb, so it's already 10 years later than Tolkien placed it. So, even if Gondolin is constructed in the exact same amount of time, we're going to be pushing the years back a bit. Which means, yes, it's going to be less than 200 years later when Aredhel leaves.
Which is...fine. What we wanted to avoid was to have Gondolin *just* built and Aredhel already going 'I'm bored!' and wanting to leave. Separating those events into not only different episodes but different seasons creates a feeling of time that flashing '200 years later' up on the screen does not. So, okay, what if it were 150 years later? Would Aredhel's restlessness feel any more fake? I don't think that's the issue.
Turgon will be moving his people into the newly-constructed Gondolin in Episode 12 or 13 of this season. We can't have it happen any earlier. That's...done. For thematic reasons, we need to establish Turgon in Gondolin in all his isolated glory as part of the Season Finale. Episode 13, the escape of Glaurung, takes place in FA 260. Therefore...we are implying a late construction of Gondolin here.
We don't have to introduce the explicit time unit of 'yen' - that might not be helpful, and better saved for next season. But we do have to indicate that much time has passed in peace with no attacks from Morgoth. Angband has been quiet for not just a little while, not just a few years, but for *two centuries.* We need that to be remarked upon in some way that clues the audience in. Otherwise - it's going to look like the elves built cities overnight, or something is just going to feel weird/out of place with the last couple of episodes in the season. We can have Celeborn's sister remark on how long it's been since she last saw Celeborn at his wedding, if we want to. I'm not really concerned with how or when the passing of time is brought up, I just feel that keeping it implicit while including a 200 year timeskip is a likely source of confusion that we might need to address more explicitly.
We've seen timeskips throughout the season. Years are passing, different episodes can take place in different seasons, etc. I'm not worried about the audience not noticing that Idril grew up (for instance) or thinking it magic rather than the more obvious 'time passed.' I think that, in general, an implicit timeline is fine, and therefore the amount of time passing is however much time seems to pass in various episodes. No one in the audience knows that the Mereth Aderthad is in year 20 or that the battle is year 60. Okay, we can make that work and live with it.
But if the season finale simply feels like '10 years later'....I feel we might be losing some of the story, and should probably be careful to clue the audience in. And, I recognize that insisting that Glaurung escape in FA 260 is part of the issue here, but we do need to give him time to grow. We don't want to imply that Morgoth can grow dragons super quickly, either.
Because, it's not just that time is passing. This is Fingolfin's Leaguer in effect. This is the Siege of Angband. This is a 400 year long peace interrupted only by Glaurung's escape. And sure, we all know that's not going to last. We know we're moving inexorably onward to the Battle of Sudden Flame and that this peace is temporary. The narrator of the Silmarillion knew that too, and took pains to point out how this plan wasn't going to work as soon as it was introduced.
But, for the elves living through it? It's 400 years of peace. That is an accomplishment and it does mean something. It doesn't mean what Fingolfin thinks it means (lasting peace and safety), but we have to justify Fingolfin's viewpoint, too. We've gotta lull the audience into not expecting what's coming (at least not entirely).
So, a better question than 'how do we show time passing?' might be 'how do we depict the peace accomplished by the Siege of Angband?'