Load html text in WKWebView

I use this code to load my html file with text in WKWebView:

do {
   guard let filePath = Bundle.main.path(forResource: "\(readBookNumber)", ofType: "html")
       else { 
           print ("File reading error")
           return
       }
   var content =  try String(contentsOfFile: filePath, encoding: .utf8)
   let baseUrl = URL(fileURLWithPath: filePath)
            
   content.changeHtmlStyle(font: "Iowan-Old-Style", fontSize:  UserDefaults.standard.integer(forKey: "textSize"), fontColor: textColor)
   webView.loadHTMLString(headerString+content, baseURL: baseUrl)
}
catch {
    print ("File HTML error")
}

and this code to load the page where the user stopped reading last time:

self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad"))

I use code for loading last page in this method:

func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
         self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad"))
    }
}

At first I used deadline: .now() + 0.1, but that didn't work. Because the last read page was loaded initially, and after a few seconds I see my text on the first page. I change it to deadline: .now() + 0.5 and the text loads fine from the last page read. Its was 700 pages. But now I want to load another text with 1700 pages. And I have same problem like first time. I can change deadline: .now() + 1.0 and my text will load fine. But I think this is not the best solution. I run it on my iPhone X. But maybe if I run it on iPad mini 2 I should change deadline: .now() + 10.0 because iPad mini 2 not very powerful. How to solve the problem?

Update based on @DPrice code:

If I use this code:

override func viewDidLoad() {
    super.viewDidLoad()
    webView.addObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress), options: .new, context: nil)

....
}

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if (keyPath == "estimatedProgress") {
        if webView.estimatedProgress == 1.0 {
            self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad\(self.readBookNumber)"))
        }
    }
}

I have same bad result like in my code.

But if I use this code:

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if (keyPath == "estimatedProgress") {
        if webView.estimatedProgress == 1.0 {
            DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
                self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad\(self.readBookNumber)"))
            }
        }
    }
}

Everything works fine. And my last page loading fine. But it does not solve the problem in my question.

from Load html text in WKWebView