First, make sure you are using Arduino 1.8.9 or 1.8.13. Some versions, especially 1.8.11 and 1.8.12 had many bugs on MacOS (affecting all Arduino users, not just Teensy). Fortunately version 1.8.13 fixed those problems with Arduino, so I recommend getting 1.8.13 so you're not also struggling with those problems.
You should also be aware Teensy Loader had a (now fixed) bug where the newer build for Catalina would not run on MacOS 10.13 (but did work on 10.12 and 10.14). That issue was fixed in Teensyduino 1.53. The older installer-based build which never works on Catalina, as far as I know, has always worked on 10.13.
Regarding this:
All of it gives me the same Unable to find Teensy Loader error, despite the Teensy Loader opening when I click upload or compile.
Please understand the ability of the Teensy Loader window to appear has nothing to do with whether Teensy is actually connected to your Mac. It's just a program on your Mac, like any other. It's just like Chrome or Safari or any other web browser which is able to open even when you're not connected to the internet, but of course they can't actually communicate until you turn on wifi or plug into a network. Teensy Loader works the same way, so please understand the little Teensy Loader window appearing only means your Mac is able to run the program.
Normally Arduino should be able to cause your Teensy to automatically go into programming mode. But you should also understand that this can fail if code running on Teensy somehow interferes with USB communication. Every Teensy has a pushbutton for exactly this reason, to give you a reliable way to get Teensy back into programming mode so you can load a new program, even if the old one caused your Teensy to no longer communicate. While you're troubleshooting, I highly recommend getting into the habit of pressing the button. Later when everything else is working using manual button presses is the time to worry about why things don't work automatically. (you can waste a *lot* of time going down that rabbit hole when the problem is actually an unrelated USB communication issue)
After you've pressed and released the button on Teensy, if all your hardware is working properly, Teensy Loader should always detect your Teensy. For testing, I recommend turning off Auto mode in Teensy Loader, so it doesn't immediately reprogram your Teensy. That gives you time to see what's happening, and you can click the toolbar buttons to send your code and to reboot your Teensy (auto mode does these for you as fast as possible, the moment a Teensy is detected in programming mode).
Focus first on whether Teensy Loader (not in Auto mode) detects your Teensy after you've pressed the button. While holding the button down, or when you unplug the cable, it should show nothing is connected. Just after the button press, it should display your Teensy model in the window. If that doesn't work, there's no point fiddling with Arduino and other software, as something isn't working between Teensy and your Mac.
When that doesn't work, bad or charge-only USB cables are almost always the cause. Of course it can also fail if your Teensy is damaged. On Linux it can fail if you don't have the udev rules installed, but on Macintosh and Windows Teensy Loader should always be able to detect the device. Since its HID which is built into the OS, driver problems are very rare. USB charge-only cables are by far the most common problem when no communication is possible.