Download and install the latest .NET SDK here.
Make sure that the dotnet command works in your preferred shell.
Open the .csproj file for your mod and add the following properties in the PropertyGroup setting:
<PropertyGroup>
...
<RunWorkingDirectory>/Applications/Stardew Valley.app/Contents/MacOS/</RunWorkingDirectory>
<RunCommand>/Applications/Stardew Valley.app/Contents/MacOS/StardewModdingAPI</RunCommand>
<RunArguments>-mode dryrun</RunArguments>
...
</PropertyGroup>
Open a terminal at your project folder and execute the following command:
dotnet watch --project <project-name>.csproj run
Now, any changes made in your project should trigger a hot reload in Stardew Valley. (press CTRL + R to force hot reload)
*You CANNOT debug with hot reload on macOS.