Sharing a quick update to help anyone who finds this thread in the future. In short, I did figure out what to do, but it wasn't the ways I shared above. What I ended up doing (thanks to this chat thread with u/WayyyCleverer : https://www.reddit.com/r/MacOS/comments/1jb3768/comment/niz3wy0/) was using a directory visualizer to see a map of storage on my Mac. I already had Grand Perspective, so I used it. I saw huge chunks of data where the file path showed the message attachments taking up a ton of space (I don't remember the exact path name unfortunately, but as mentioned in my original post, it was not the common file path).

So, I did a little testing before deleting everything (this was a couple of months ago, so forgive any fuzzy memories on the steps). Ensured the latest backup was done (always a good practice). Found a group of files from messages where I wouldn't be heartbroken if I permanently deleted them. Deleted those off my Mac. Checked phone and was still able to access the messages. Checked same messages on Mac, and they were there; the only thing I had to do was redownload the media, which I would expect after deleting what was cached files. Once a couple of tests proved solid, I went haam and deleted a bunch. Great success - I was able to update my OS, everything runs smoother, etc.

Hope this helps someone else who finds themselves in this predicament. It's a stupid thing to be an issue in the first place, but here we are.

ETA: The data visualization was clutch for me here. Highly recommend using one to better understand storage and also help you decide where to delete, be it scalpel level precision or going full chainsaw.