I agree that there are many moving parts here. However breadth-first exploration is rarely a bad thing in problem solving and thus laying out the possible directions (no matter how unlikely they seem) is beneficial. Timings and unlikely events are usually the cause of many problems after all.
TestObject tiny = makeTO("#tiny-react_24806645131568596486738_ifr")
String js = '''
var doc = document.querySelector("#tiny-react_24806645131568596486738_ifr").contentDocument;
var p = doc.querySelector("#tinymce p:nth-child(2)");
p.innerHTML = "<b>Hello, World!";
Also, If you want to copy/paste the text to the editor you need to install the plugins. I am using pasteitcleaned for this thing. It handles copy/paste from Word, Excel, PowerPoint, PDF, Websites, Paint, etc. You should try it once.