However, we have historical records of the different parties at the time. So you can read the words of multiple Congressmen, how they explain the law, the debates over the law, etc. That is what I mean by original intent. It isn't merely definitions. It isn't only the words in the law, though that is the first priority. It isn't even the words of one politician commenting on the law. It is the full body of commentary on the law when it was drafted, debated, voted on, and later explained.
You are correct. There is a lot of material that we often refer to as "legislative history" - committee reports, bill analyses, statements by members of Congress, debates, and a whole host of other things. There are problems with relying upon them though.
First, they usually are not written by the members anyway. They're written by committee staff, legislative counsels, and often, lobbyists. Do most members even read them? No. When you get to debates, you'll get the actual members' words, but whose words do you rely upon? At least a quorum of each body votes on a bill, but how many publicly debate or comment on the bill? Most do not.
Second, what if the legislative history is inconsistent? Not everybody in Congress agrees on what something means, and they definitely have different comments on it. That happens with legislative history. If a judge relies upon it, it is not hard for that judge to cherry-pick the legislative history that supports his preferred outcome or rationale and disregard the history that does not. In short, it is very easy to give an illusion of objectivity with legislative history when the judge's inquiry was actually subjective. Basically, it leads to latent judicial activism.
Third, if courts rely on legislative history, it invites shenanigans on Capitol Hill. Specifically, it invites members to pass ambiguous but politically popular language but then dump nameless and faceless hints and clues about interpretation into the legislative record that encourage a politically unpopular result in the judiciary which doesn't account to the public.
I will say I would give more weight to the actual author(s) of the law, than what others said about it. If a Congressman writes a bill, explains what he means and debates it with others, I am going to focus on that since he is the source.
I used to believe this until I brought the issue up with my legislative power and procedures professor. Like you, I asked who would be a better authority on a bill than its author. He directed me to the the bicameralism and presentment concepts from Article I - not to suggest that he isn't an authority on the bill but to put that author into context of the broader legislative context and therefor, why his opinions shouldn't matter as much as one might assume.
The House and Senate (who hold the Article I legislative power, not an individual member) only concur on the actual text of a bill - not on committee reports or even members' comments. The same is true of the President's involvement. He's signing a bill that codifies statutory language. That's it. He's not signing a committee report and has no bearing on it. Accordingly, the author's comments simply don't have much bearing on the comprehensive intent of the lawmaking body and the President who signed it into law.
What I have to chuckle at a little is when my position is framed as the work of liberalism and judicial activism. The most famous adherent to my view of textualism and the rejection (and often with great contempt and ridicule) is Justice Scalia. In fact, it might be his most significant contribution to American jurisprudence. Justices Thomas, Alito, and Gorsuch largely concur.
Justices who rely heavily on legislative history don't tend to be the most old school liberal justices (the Warrens, Brennans, Marshalls, etc.), because they didn't feel the need to put on a pretense that they weren't just making **** up. It's squishy guys like Breyer, Roberts, Kennedy (though he evolved quite a bit under Scalia's influence), and others. It's not the work of reliable conservative justices.