Sadly I've had this problem when I first tried to update the system in 2018 when I first got it. It kept trying to update Windows in the background and kept trying to restart only to fail to install the sys updates. This kept happening to the point which caused a hinderance to my daily use so I turned off Windows Update.it sounds to me like a clean install is the only way you will get past the problem it seems to have with that restart.
Microsoft changed how updating worked in the past but I am pretty sure that was before 1803. Jumping 2.5 years of updates in one move might be too much for your windows updater to handle at once... Fresh install won't give it any choice.
Would something like a Reset be enough? I have ordered an USB but it won't arrive here until the 25th.
Yeah, it's all good to post the results here on the post. Sorry if it seems like I'm hurrying you, I just wanted to clarify if I came off that I didn't want it on here before.