This whole thing is a misunderstanding of how the US system works. None of this is a legislative function, it is a determination by a rule making body. While I agree that the courts are often overworked and we need to vastly expand the number of judges to make sure cases are heard on time, this isn't the root of the reason the functions of the NLRB or the SEC aren't handled by the judiciary. It's that they're simply not judicial.

Instead, congress has designated some rule making authority to a group of experts, this group of experts is an executive body doing executive work, so it's normally under the executive branch. This body makes rules that have to follow any laws set down by congress, which is one check, and the courts can settle disputes between someone who has to follow the rules and the rule making body, or congress and the rule making body, etc. which is another check. The rule making body makes the initial determination on whether it's rules have been followed not because they're taking on some function of the judicial, but because they're the experts who know what their own rules mean. If they get it wrong, willfully abuse that power, etc. the judicial is still there. It's separation of powers all the way down. If you removed them from the executive and put them in the judicial you'd be giving some judge rule making authority and the ability to interpret those rules later. The whole point is to separate that between two branches of government.