Is it possible make Preview not enhance thin lines?
Preview's preferences used to contain a setting "Smooth text and line art", but it was removed from the app (around Sierra/High Sierra, I think, but can't be sure).

The plist data key associated with the preference was PVPDFAntiAliasOption. It's possible that this key will still work. You may want to try:

defaults write -app 'Preview' PVPDFAntiAliasOption 0


and then relaunch Preview. If this doesn't work, then Apple have indeed decided to permanently hold lines at any zoom level, and you'll need to use another PDF viewer for this purpose.