This is a good observation, and it holds up. Boldface questions are asking about the role each portion plays in the argument, so if you cannot name what a bolded piece is doing, the answer choices turn into a guessing exercise no matter how well you followed the reasoning.
I would push your labeling one step further. Beyond what each portion is, tag whose side it is on. A piece of evidence can support the author's position or the position the author is arguing against, and boldface questions lean hard on that distinction. Same with conclusions. The bolded statement might be the author's main conclusion, an intermediate conclusion the author uses to get somewhere else, or the conclusion of the view being rebutted. All three can read as "the author's opinion" if you are sorting only by statement type.
What makes this reliable is finding the author's actual conclusion first, before labeling anything else, then placing each bolded piece relative to it. Once you know what the author is ultimately arguing, the roles fall out. Worth doing on every boldface question you practice rather than only the ones that stump you, so the labeling becomes automatic.
quasivoluptatem
The part nobody explains well Boldface CR questions kept beating me and I couldn't figure out why until I noticed the actual gap: I wasn't struggling with the argument, I was struggling to correctly label what each bolded chunk even was — fact, supporting detail, or the author's own opinion/conclusion. Once I started explicitly tagging each boldface portion with one of those three labels before touching the answer choices, the question type got a lot more manageable. If boldface CR feels randomly hard, check whether it's actually a labeling problem before assuming it's a logic problem.