Hi Adrian. I'm Greg, 10 years awarded Windows MVP, specializing in Installation, Performance, Troubleshooting and Activation, here to help you.
Where are the results of the steps I gave you to try and asked to report back results? This tells me what to suggest next.
Have you uninstalled Bitdefender including with it's uninstall tool here:
https://www.bitdefender.com/consumer/support/an...
Thanks for clarifying the error you're getting. Try the fixes here:
https://www.thewindowsclub.com/windows-10-updgr... (Avoid ad links which intrude into editorial copy, especially avoid Restoro and Reimage ads)
https://www.howtoedge.com/fix-error-0x8007042b-...
https://www.majorgeeks.com/content/page/upgrade...
https://www.youtube.com/watch?v=ioVURDLaLCI
https://www.youtube.com/watch?v=QPLayXx2kw8
Also, may I ask why you're in a hurry to get Windows 11, especially if you're not willing to reinstall your Apps due to licensing issues which mostly do not exist any longer.? I am the author of the tutuorials which popularized Clean Installs among consumers over the past decade. No software provider wll stay in business who does not make it easy to reinstall because consumers want to do this whenever they please and will not tolerate disruptions. Perhaps you can share your concerns more in depth so I can help you since I have knowlege about most all of these, and many are covered in my latest gold standard Clean Install tutorial which is being updated now for Windows 11: http://answers.microsoft.com/en-us/windows/wiki...
Keep me posted on your progress as I will be here to help until the case is resolved.