A read-thru: Wittgenstein's Blue Book (Sec. 14: Variety of criteria and the place of pain)
Immediately after your quoted sentence he says:
"It is not new facts about time which we want to know. All the facts that concern us lie open before us."
Let's replace "time" with anything, say a tree or an idea. It is not the case that "all the facts" are open to us, only those facts which we are of aware of at the given time and (crucially) those facts which we may have no access to.
We are limited on our observations about trees relative to the capacity of our senses and the capacity of our cognitive components.
This applies to our mental powers, or mind as well and much else. He is problematizing something like mental mediation here, which is not clear to me what the problem is.
This is an important connection than my merely trying to record the aghast commonly felt at what is seen as removing the self (just, as an object), when he is just following through the categorical error of the ‘strong temptation’ of causality. I would only add that we would be “standing on the outside trying to look in” to ourselves as well if we imagine we can “look into” our own casual object (agent, “self”). Not to move further from the text but to place this in company, the PI will treat the other as opaque and talk of boxes with things hidden, etc. — Antony Nickles
I see him saying that we are not concerned with the "causal connections" here, not that they are a category error. We can discuss this if we want, rather, we are choosing not to do so now.
I would agree that we can't - despite (some of) us wanting - peer more into the nature of the self than something like what Hume describes. In this part, I do agree with the example and the general outlook.