I've followed the instructions in the following forum post but it did not stop from auto starting up for me:

https://forums.opera.com/topic/24685/disabling-auto-startup/80
"If anyone still have problem with Opera auto startup with windows then do nex to stop it: Go to oper instalation directory open assistant and change browser_assistant extension file to be anything but just not exe. i have put browser_assistant.ex and opera didnt startup with windows."

There was also this post stating that this was a known bug back in 2018. Also mentioned things about messing with the registry but the registry was matching more than one folder inside.
https://forums.opera.com/topic/25203/opera-autoruns-at-windows-boot-with-4-tabs/33

Any thoughts on what to try next?