bad RAM, i would doubt that considering the problem only started after the 1903 update.
for me, i got BSOD BAD_POOL_CALLER very 2 minutes or so, and the source of that was Dell Support Assistant app that was installed. un-installing it solved the problem immediately. luckily it managed to say on long enough for me to get the dump file and see what the issue was.
Dell have now only just confirmed there is an issue and they advise to remove it. may not be the same problem the OP has, but investigation should start with similar apps first.
Speculation on my part, based on past experience that the DISM cleanup tool didn't work with the 1803 update. The update had corrupted the user profile as detailed elsewhere. I've had 1903 for about 3 weeks now and have had no problems with it so far. Windows Update has behaved strangely on the odd occasion (an unlisted update upon restarting). Typing tree in the root directory (C:\) in cmd reveals a number of version numbered packages, leading me to believe that Microsoft slips the odd update in with them.