It’s still A shame though to see each individual character from the original 8 get big new content except for Luigi and Yoshi.
I think you're missing the point my guy.
They got content. New content. Look at their movesets, among other things.
Luigi isn't a fill on clone of Mario anymore. Dude's final smash changed in Sm4sh compared to Brawl's, has a tether grab, and can do combos that Mario can't do. Likewise, being the unique lightweight fighter he is, Yoshi's FS changed completely in Ultimate compared to Sm4sh and Brawl. Both Luigi's and Yoshi's current FS's represent their spinoff series quite well.
Now let's move away from the movesets and look at the other factors:
We have a Luigi's Mansion stage in Ultimate and even though it never returned (to my knowledge) Sm4sh got a Yoshi's Wooly World stage.
Additionally, games from Brawl up to Ultimate had Luigi's Mansion content.
Finally, Yoshi's got 4 stages for his series in the game. That's a great number of stages for a series with 1 fighter.
Other than MAYBE lacking Poochy, and/or Kamek and or Birdo as fighters, the Yoshi series is repped just fine.
As is the Luigi's Mansion side series, even though keep in mind Luigi's classified as a Mario character because in the end, he mainly is just that.