Can't open Sims 4, stuck on on "Verifying 'The Sims 4.app"

See here for what a hard uninstall is: https://help.ea.com/en/help/origin/origin/manually-uninstalling-origin/#hard-way

The message I put in quotes is the message the shows up. I am attaching a screenshot. It used to show up when I was able to launch the game, it was a pop up that came up when I launched the game and after it finished loading the game would launch. Now it just never finishes loading. I have had it sit and wait for 30+ minutes and it never loads. Sometimes it gets very close to the end of the loading bar and then just starts over again. Sometimes it gets to the very end and just sits at the same spot forever until the app crashes. 

So the question of where it is installed is kind of complicated. The whole reason I stopped being able to open the game is because I was having a lot of performance issues playing it installed on my laptop so I wanted to move it to an external drive, and I followed some instructions I found on reddit and probably did something wrong, because I wasn't able to open the game after moving the files to the drive and started getting the endless "verifying" loading screen. At that point I tried to move the files back to my laptop, thinking I'd just manually uninstall them, then change the download location for new downloads to the external drive and download them all again from origin. But when I moved the files back to my laptop I tried opening sims just to see if I could again, and I was still getting the endless "verifying" loading screen. Over the last two weeks of communicating with EA support I've tried deleting and reinstalling the files on the external drive and on my laptop, and no matter where I install them I still get the "verifying" screen that won't stop loading if I try to launch the game. 

Just now I checked my permissions and you were right that origin and sims 4 didn't have full disk access, but changing that didn't let me open sims 4 with the files installed on my laptop. I then tried trashing every file listed in the link you sent me, trashing the old sims 4 files, reinstalling origin, and opening origin and reinstalling sims 4 on the external drive. The download is currently stuck on the same "verifying" loading pop up. Currently Origin has full disk access in my preferences, but I can't give sims 4 full disk access because it's not currently installed either on my laptop or on my external drive.

Ultimately, I would like to have sims 4 and all my purchased packs installed on the external drive, but I am open to anything at this point just to get the base game to open again, and then would be fine with figuring out how to get all the files on the external drive after if that is easier. 

Thank you, this is really helpful info. I'm usually good at following instructions on tech stuff but am not that skilled/knowledgable on any of it myself. Before I removed all the sims games from my mac I still had some storage left, but very little. However, based on your response I think you're correct moving the files to the external drive probably wouldn't have fixed anything. The performance issues started when I downloaded a few new packs, so it sounds like the issue wasn't about storage but that my hardware wasn't robust enough to handle running the game with that many packs. Am I understanding right?

In that case, would the best solution for being able to continue playing the game on my laptop without the same performance issues be to still have the game and packs installed on my device since I have the space, but not download all the packs I was previously using and leave some uninstalled so the game doesn't overwhelm my hardware? (I'm planning to upgrade to a gaming PC at some point, just looking for an interim solution that allows me to play the game without killing the mac)

Yes, when I said get all the files on the external drive I meant all my packs and kits. I've gradually accumulated a lot of them and the performance on my mac was okay for a while as I kept adding on, but after the last two or three expansion packs there started to be issues.

It's a 2018 macbook pro, full specs in the attached screenshot.

At the moment it says there's 74.01 GB or storage available, but there are no sims apps currently installed. I think before I tried moving everything to the external drive there was around 20 GB still available, probably a little less.

It's a 500GB Seagate slim for mac portable drive, the model number is SRD00F1. I don't know what the connection type is called, but I connect the drive to my computer using the cord the drive came which has a micro-b connector going into the drive and a usb output, and I connect the usb connector to the apple usb to usb-c dongle and plug the usb-c connector into my computer.