Perception is an opportunity for the GM to introduce new challenges
Spotting an object is a hook to a new challenge. If your player can trivially spot many fine details, don't bother rolling. Just tell them what they find. However, don't give away the farm - only tell them things the character is capable of finding out. Everything else is part of the story and succession of challenges you are building.
For example, imagine a character that is investigating a scene where a wagon or caravan was attacked. Perhaps they find arrows or arrow heads at the scene. Instead of saying "you find some goblin-made arrowheads, they probably came from the northern goblin tribe you heard about", tell them they have arrow heads and let them do the follow-up work.
Sure they found some arrows, but are they capable of telling the difference between goblin-made arrows and any other? Can they tell whether they were fired by the attackers or defenders? Can they surmise which tribe of goblins would create these particular arrows? Do they have enough social knowledge to understand why goblins would attack a caravan? Do they know enough about business or wayfinding to know why the caravan was there in the first place?
A successful perception check is just an opportunity to learn more about the world. Run with it.
Solo Adventures are more tailored
Although you explicitly asked about the challenges with the Observant feat, I suspect some part of this problem is also about the unique challenge of solo adventures. With an ensemble party, you create challenges that will challenge or intrigue the party as a whole and only occasionally focus on an individual character. That's part of sharing the lime light.
In a solo adventure the entire adventure is tailored to the experience of a single character. It's okay to tailor the plot, challenges, and specific details to exactly what that player is interested in and that character can support.
In this case: be a fan of the character. Let them observe all kinds of details. And create fun adventures that utilize this skill as an opportunity to further challenge. Instead of fighting their strengths, play to them.