I don't really get all the complaining about Avalance in 4b. Considering Ava was missing for 1/4 of the episodes, they actually had a lot of focus?
4x9: Build up to the fight, the tango, the constant back and forth and the big argument at the end.
4x10-11: Ava's missing but Sara mentions her more than once in 4x10 and is clearly very upset because she assumes Ava's ignoring her. And in 4x11 she has a sex dream about Ava when they could have easily used that whole lust spell set up and Sara assuming they were broken up to have her flirt with anyone. All she wanted was Ava.
4x12: Sara literally puts her life/soul at risk to go to Ava's purgatory without any hesitation. Yeah, the Idea-themed purgatory wasn't exactly excitement but the whole thing was basically affirming their commitment to each other.
4x13: Date night, book club, fighting together, lots of cute scenes and Sara very obviously making an effort while she had the chance.
4x14: Mostly separated by plot, but Sara tried to contact Ava as soon as they were in trouble and once Ava was herself again, she went right to Sara.
4x15: The two of them held captive by Gary and the Fairy Godmother and trying to work together.
4x16: They gameplan together and Ava's completely with the Legends, taking the responsibility for filming everything as part of their plan while Sara leads the fight against Neron and Tabitha.
Of those episodes their plot in three of them was largely about the relationship itself (9, 12 & 13) and the two of them working together was a general focus of a few of the others. Nate and Zari had a lot of focus because that relationship came out of nowhere and they were trying to go from 0 to 'sacrifice myself trying to save you' in less than half a season.
There is no main relationship on Supergirl right now (and there wasn't really last year either), Arrow's finale was the goodbye for his love interest, and in Flash Barry and Iris lost their child, so of course those were going to be big plot points.
Ava bonding and working with the Legends is part of her developing relationship with Sara. The reason she even bonded with Mick in the first place was because Sara was trying to bring her more fully into the emotional family on the Waverider, so that was a sign of how she is consciously becoming a part of Sara's extended family. Particularly with Mick, who is one of the originals and who doesn't accept or bond with newcomers easily.
Sara is still the main character, but it's an ensemble show and like Amaya leaving last season or Stein's death, there are always going to be other characters having the focus in big moments and it doesn't get much bigger than the ones that Ray, Nate and Zari had there at the end. And I'm not at all bothered by the face that it was Nate dying/not dying or that Zari was the one who fundamentally changed because Sara has died so often it's a joke at this point and Ava nearly died just a few episodes ago. And I certainly don't want either of their characters to forget everything and change their entire history and memories of each other.
If you want them to manufacture drama between them just so they can have moments in all the episodes, I'd say what sets Legends apart is that they generally don't do that and the relationships and characterizations are better for it.
Other huge things happened in all the other shows as well, big plot points for characters that weren't the 'main character or couple' like Cisco giving up his powers and Lena learning the truth.
Sara lied to Ava in the beginning of the season because she wanted Ava to see her as a hero, then finished up the season having to prove herself in front of billions of people with Ava there to be the literal lens through which everyone was able to see Sara at work.
