Increase security in SQL Server with known login that cannot be changed or dropped?

Question

I have a server type software connected to my local SQL Server and it has a hardcoded SQL login/password(also requires sqladmin role) inside that I CAN NOT change. As I have to use this software, I had to create this login in my SQL Server. Now the issue is, this hardcoded login info is well known on the internet, ask google with few keywords and clicks you can find it. Also I CAN NOT close 1433 or any SQL Server port on firewall as my clients need to access to my SQL Server for data and functions.

So to prevent people who know this infomation and want to use it against / break into my SQL Server. I did below security measures:

1. Turned off SQL Server's remote connection function
2. Create an logon trigger to allow only local communication for this login

Now the question is, can someone spoof as localhost to send me an packet with SQL command such as DROP TRIGGER etc. to break in?

If so what other counter measures I can use? Heaps thanks!

Trigger as below

CREATE TRIGGER prevent_ip 
ON ALL SERVER
FOR LOGON
AS
BEGIN

DECLARE @LoginName sysname
DECLARE @LoginType sysname
  
SET @LoginName = ORIGINAL_LOGIN()
 
IF(@LoginName = 'HardcodedLogin' AND (EVENTDATA().value('(/EVENT_INSTANCE/ClientHost)[1]','nvarchar(128)') IN ('<local machine>')))
BEGIN
RETURN; -- Login Authorized
END

ELSE IF(@LoginName IN ('SecuredLoginForClients'))
BEGIN
RETURN; -- Login Authorized
END

ELSE            
BEGIN
ROLLBACK; -- Disconnect the session
END

END