Setup

Common should have the following files:

└───Assembly-CSharp_unpacked
    └───Assembly-CSharp_unpacked.csproj
└───mu3
    ├───mu3.exe
    └───BepInEx
├───Assembly-CSharp-firstpass.dll
├───mscorlib.dll
├───UnityEngine.dll
└───UnityEngine.UI.dll

The unpacked assembly can be found here and unzipped with dotPeek/ILSpy.

The DLLs can be copied from mu3/mu3_data/Managed.

mu3 can be symlinked/junctioned.

Once Common is set up, run dotnet restore in the root directory and in Assembly-CSharp_unpacked, and open the solution in MSVS or VSCode or whatever. The DLLs will be copied to mu3/BepInEx/monomod post-build automatically.

Best practices

Everything must be independent from each other, but everything must also work in conjunction with each other and with the recommended mods from the old modpack.
By convention, the bare patch_ format is used unless it's not possible.
Put source files in namespace subdirs.

Troubleshooting

The type or namespace name could not be found

If you are getting a bunch of these errors, something may be wrong with your decompiled assembly, or my setup is wrong. Someone knowledgeable with dotnet could perhaps explain it (I'm just a tourist). A potential workaround is to add the unpacked assembly DLL to Common and reference it in project files:

<Reference Include="Mu3Assembly"><HintPath>..\Common\Assembly-CSharp_unpacked.dll</HintPath></Reference>