Sorry I should have broken it down.
それ - “that”
だけ - Can mean “only, just” or “nothing but” depending on context.
の - I said “possessive” but I think it’s more properly “of” in this case
こと - “thing”
なのでは - “when it comes to”. This phrase is three N5 grammar points in a row. な as a stand-in for だ; ので to mean “because” or “since”; and では for the “when it comes to” part with the other stuff just being connectors.
ない - “negative”
だろう - “I think” or “I suppose” but also implies something of the listener so I just used the English tag “right?”
か - “?” Basically the question marker.
And even with all that, many of those points are debatable. Take the の for instance. The first one is the “possessive” or connector form and the second one is part of a set phrase that’s taught as a grammar point. You can either think of them as two different の for each use or the same の used in two different ways and both views are valid.
I agree.
But, like I said, only for clearly defined sentences like in your example. In many cases, the entire sentence will be more than the sum of its parts where you have to take a holistic view of that particular sentence and even surrounding sentences in order to glean meaning.
Like @NicoleIsEnough pointed out, the book clubs are a good resource for that as the discussion involves the kind of breakdown you’re looking for.
You may also want to check out the Short Grammar Thread where you can find discussions on things like what exactly なのでは is doing in the sentence.
I think it would be great if there were resources out there like you want and maybe we’ll get lucky and someone will post one. 
Edit:
@Aaydothaytch I actually just remembered about Game Gengo:
He does the kind of breakdowns you may be looking for.