I was up against a frustrating issue while attempting to upgrade a project from 2022 LTS to 2023.3.0a6. I’m sharing the solution here for anyone else that runs into this.
Opening my project kept failing during import (fresh / with no Library folder in project), it was just suggesting Safe Mode. The errors didn’t have line numbers or anything, so I just tried random stuff like adding folders as safe for antivirus, restarting computer and so on… nothing helped. Then I installed 2023.1.14f1 and tried that instead, figuring if it worked I could try upgrading again to latest from that, but had the same kind of problems:
If I created a new project, everything was fine… so I opened mine in Safe Mode again, dug through Project Settings and happened upon the cause, which was that my project used .NET Standard but there’s a new Editor Assemblies Compatibility Level dropdown now in 2023.x which was on Framework, so once I set them both to the same (initially trying Framework, which worked, but then switched both to Standard and that worked too), it starting importing everything without errors. I could then enter Play Mode etc.
So basically all you have to do is click Safe Mode when it asks, then go to Edit->Project Settings->Player, and change your Editor Assemblies Compatibility Level to the same thing as your Api Compatibility Level setting (the dropdown above it). Then wait a while as it compiles and imports 
I’ve also retested this solution from scratch (again going directly from 2022.3.10f1->2023.3.0a6) and it worked. Suggestion for Unity: as part of upgrading our project, when previous editor version didn’t have that setting (because it was 2022 LTS for example), automatically set this based on Api Compatibility Level?