No matter how I try:
SATURATION/1/300
SATURATION/300
They are both instant instead of giving saturation for 5 minutes.
Is there a way to make it longer?

I tried to compensate this by giving players a negative Hunger effect:
- HUNGER/-10/300
which ends up as ordinary Hunger V effect.
Is there a way to apply negative levels of an effect?

We only support the Potion Effects of here https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/potion/PotionEffectType.html
And sadly no negative effects, is that even possible?

Wiki states at the Hunger page: "Negative levels decrease food exhaustion, although they do not increase saturation or the hunger bar."
/effect in Minecraft does not allow negative levels, but they must have gathered that piece of information somehow.

nvm. spigot must have screwed saturation up, since "/effect give @p minecraft:saturation 300" does work. It is 300 ticks and not seconds, but still fine ...

Is there a way to workaround the SATURATION/1/300-1200 effect? For example by having quality specific commands to run, like:

    servercommands:
      - + effect give %player_name% minecraft:saturation 300
      - ++ effect give %player_name% minecraft:saturation 750
      - +++ effect give %player_name% minecraft:saturation 1200

I read in the code that it does not support it, but parsing player / server commands exactly like the lore, would allow for this and many other shenanigans. Would you consider adding that?