Why can't this thing accept a circuit connection for enable/disable? I had to wire this thing up with a power switch based on my circuit condition, that's very annoying and extremely unnecessary.

Yeah, it totally makes sense to do that. I did not implement it, because I didn't find any way how to do that. If you know any mods that introduce new buildings with circuits enabled, please direct me to them, so I could take a look how they implemented it and perhaps do it the same way.
I am glad that you found a workaround with power switch. Another workaround is to use pumps and enable/disable those, that's how I use it in my factory.
But I would like to add the circuit condition on the building directly in a new version.

https://lua-api.factorio.com/latest/classes/LuaGenericOnOffControlBehavior.html#circuit_enable_disable This is the documentation page, you can search for Circuit and see how to extend them.

You could also just make a deep copy of pump (since it is one) and then replace the core tick logic with your own and the graphics, that would inherit its functionality to the circuit network.

Duplicate post.

I implemented it, you can download version 0.1.3, now both offshore dump and grinder can be configured by circuit network. It was much simpler than I thought, I was just missing wire connection settings in the prototype definition, that was literally just one line that I had to add. Thanks for pointing this out and have fun!

Thanks, I don't need the power switch anymore. But as I was wiring up the decider combinator, I saw this? https://i.imgur.com/bmJnv0U.jpeg

I don't know if you intended those to show up in the circuit network, I have no earthly idea what possible use that information could be to the circuit network... You could simply just deduce the inverse of it, by knowing how much original fluid is being removed...

Also, why or even how would you be able to dump thruster and oxidizer? They can only be made in space, and there's nowhere to dump it...

You can dump them in space, just create the offshore dump at the edge of the platform, similar to inserters dumping items. It can be useful if you want to prevent factory halting when some output is full.
The offshore dump is technically a furnace that has those "smelting" recipes that take a fluid on input and give nothing on output, so each dumping has to be represented. I did not see any strong reason to exclude them from the circuit network, so I left them there, it feels consistent to the barrelling recipes that are also there.
Also now that the offshore dump is connected to the circuit network, it outputs those signals as "recipe finished".