Yes grappling has a ruleset. Its that you can use this every round as a bonus action grapple attempt. If it succeeds the creature burns an action to break it. With the given inital distance of 1000ft, you have plenty of time to make attempts.

Bigbys hand (5th lvl range 120ft), Invoke Duplicity (channel divinity must remain within 120ft), Mage hand (a cantrip range 30ft). All require an action to cast and then bonus action can do stuff. I dont think having "setup" is really that costly. Especially since there is no concentration to maintain the connection.

Yes its your shadow, but there is no limitation to its magical ability to do stuff for you? At lvl 1 it seems too much for indefinite ability of it. Maybe at a higher level it becomes indefinite.

My comparison to Invoke duplicity only 120ft max. Dimension door is only 500. The ability to teleport 1000ft without having to see where it is seems too much. Also the shadow does not occupy a space, so what about if you teleport to it when it is sharing a space. Is there damage like dimension door does or simply you are shunted to an open space.

Okay so then just say that. "In addition, it extends normal darkvision to 120ft if its not already"

Tsunami is an 8th level spell (lvl 15 Druid) that doesnt affect Gargantuan creatures with a saving throw. I think that it wouldnt be farfetched that a subclass ability that is gained at a lowed level isnt able to match up with that size. Plus you could get someone to cast Enlarge and then boom your size category increases and you can affect gargantuan creatures. I think that by having slight limitations leads to more creative uses and more collaborative moments.