AppleScript Help: Wait for Safari Page to load
There are a few methods you could use to check whether the page has finished loading.  Here are a selection that I've had good results with during testing:

Document name

Monitor for the existence of the Safari document that has the same name as the title of the webpage, which (I believe) only gets assigned after the page has finished loading, up until which point it retains the name it had previously, or "Untitled" if it's a newly created document:

tell application "Safari"
    make new document with properties {URL:"https://csbsju.instructure.com"}

    repeat until the document named ("Central Authentication Service " &amp; ¬
        "| College of Saint Benedict &amp; Saint John's University") exists
    end repeat

    log "Finished loading"
end tell


The Login button

Monitor for the existence of the "Login" button, given that you can't do what you need to do until that particular HTML element has been created:

tell application "Safari"
    make new document with properties {URL:"https://csbsju.instructure.com"}

    tell front document to repeat until (do JavaScript ¬
        "document.getElementById('btnLogin').id") as text is not ""
    end repeat

    log "Finished loading"
end tell


Safari's Reload button

Monitor particular properties of Safari's Reload button, which changes according to whether a page is loading or has loaded:

Note: This was written and tested under macOS High Sierra; however, for macOS Mojave one minor edit is required. Change UI element 1 to ¬ to UI element 2 to ¬ in the code directly below.

tell application "Safari" to ¬
    make new document with properties {URL:"https://csbsju.instructure.com"}

tell ¬
    application "System Events" to tell ¬
    process "Safari" to tell ¬
    window 1 to tell ¬
    toolbar 1 to tell ¬
    groups to tell ¬
    UI element 1 to ¬
    set reload_button to a reference to ¬
        (first button whose name is "Reload this page")

using terms from application "System Events"
    repeat until the accessibility description ¬
        of the reload_button ¬
        contains "Reload this page"
    end repeat
end using terms from

log "Finished loading"


Page content

Monitor for specific text to appear within the page content.  In this case, I've chosen to monitor for all of it, but you can choose smaller elements instead.  Items that appear towards the end of the page are most suitable:

tell application "Safari"
    make new document with properties {URL:"https://csbsju.instructure.com"}

    ignoring white space
        tell front document to repeat until its text contains ¬
            "Central Authentication Service Network Username: " &amp; ¬
            "Password: Warn me before logging me into other sites. " &amp; ¬
            "Copyright © 2017 - All Rights Reserved College of Saint " &amp; ¬
            "Benedict and Saint John's University"
        end repeat
    end ignoring

    log "Finished loading"
end tell


Username &amp; Password fields

Since these fields are getting filled out automatically by Safari's password manager—which only happens when the page has fully loaded—you can monitor the text content of the Username and Password fields:

tell application "Safari"
    make new document with properties {URL:"https://csbsju.instructure.com"}

    tell front document to repeat
        set a to (do JavaScript "document.getElementById('username').value") as text
        set b to (do JavaScript "document.getElementById('password').value") as text

        if length of (a &amp; b) ≠ 0 then exit repeat
    end repeat

    log "Finished loading"
end tell