I found your modified version of the service after using the original for a while. I haven't had any issues with the original except for the desire to run the client remotely instead of popping open a RD window. However, I followed your instructions after downloading the most recent build and get this message when I run start it.

System.ServiceProcess.TimeoutException: Time out has expired and the operation has not been completed.
   at System.ServiceProcess.ServiceController.WaitForStatus(ServiceControllerStatus desiredStatus, TimeSpan timeout)
   at Topshelf.Runtime.Windows.WindowsHostEnvironment.StartService(String serviceName, TimeSpan startTimeOut)
   at Topshelf.Hosts.StartHost.Run()

The service install goes fine and if I try to run the application without creating the service it spits out

ERROR [1] (:0) - An exception occurred
Topshelf.TopshelfException: The service failed to start (return false).
   at Topshelf.Hosts.ConsoleRunHost.Run()

So basically the same thing just without the references to it being a service.

When attempting to start bedrockservice.exe for the first time with or without Minecraft Bedrock server already downloaded.

The output is as follows:

Checking MCS Version and fetching update if needed...
Updater resulted in error: An error occurred while sending the request.
Updates Enabled, will be checked in: 9909.057 seconds.
Starting service...
ERROR: Got zipfile exception! Could not find file 'D:\Games\Minecraft\BedrockService\MCSFiles\Update.zip'..
!!Deleting entire directory!! Press Enter to continue...

Checking your Updater.cs file, it appears to be failing at line 22: content = await client.GetStringAsync("https://www.minecraft.net/en-us/download/server/bedrock");

I've added bedrockservice.exe to both inbound and outbound firewall rules for good measure. There must be some other security setting I'm overlooking that is blocking this from working.

I am running Windows 10 21H1 build 19043.1165 with .NET v4.8.04084