How concise can I get the Soulbond boilerplate in a homebrew card without functional changes?
All three wordings have the same effect. The last one looks different in that it doesn't say that the bond goes away if control changes or they stop being creatures, but the rules cover that; the wording of soulbond is redundant.

With none of your wordings does the ability have a target. This has some minor effects, like allowing you to pair with a creature with shroud or protection and the not making you choose the creature to pair with until the ability resolves.

The first two could also be changed to say "this one" instead of "this creature", like the original.