FeartheGreat if you're not opposed to using cheats, the Beloved Trait usually stops relationship decline. You can add it with this cheat:
testingcheats true
traits.equip_trait trait_Legendary
I add this to all my Sims now to stop relationships from declining. My current Sim is friends with one fairy and it works for their friendship but I haven't played as a fairy to see how their gameplay affects it. If you don't want to use a cheat you just need to complete the Friend of the World aspiration and a quick way to do that is to buy a Sim the Incredibly Friendly reward trait for 2000 points before starting the aspiration.
Note: On console, cheats requiring testingcheats true will disable console achievements and trophies for that game save.
I am not opposed to using cheats as I do so a lot of the time myself, but that's what I mean when I say, "gaming the game". It's harder to keep up normal relationships with fairies compared to regular sims because the negative impact from absorption is a bit too strong.
