Pls. forget "store it as it is" all you store is the oref (a pointer to a local structure) with no persistent storage behind.

set local= dynamicObject.%ToJSON()
set classObj.Test = "my dynamicObject = "_$LENGTH(local)

so you don't get an error if you just apply  %ToJSON()  ???

 If your version is before 2016.2  than .$toJSON() would be the right method

You didn't indicate the line you get the error.
anyhow the message is clear:
Either classObj or dynamicObject is not what you expect it to be.
If you test from Terminal you can use

ZWRITE classObj
ZWRITE dynamicObject

to see what they are.

otherwise, you may comment out one line and see if it still breaks e.g.

// set local= dynamicObject.%ToJSON()
set classObj.Test = "my dynamicObject = "_$ISOBJECT(dynamicObject)

So you see what happens.  I expect:  my dynamicObject = 0

So you see where you have to investigate further

is it defined that way?
Property Test as %String (MAXLEN="") ;

ATTENTION - default MAXLEN = 50 !!!!!

This might look odd but try

set local= dynamicObject.%ToJSON()
set classObj.Test = local

 
so you see if the issue is your property  
or your dynamic Object. as there are 2 objects in the command.
the only thing %String is checked against is Length. 
otherwise. if %ToJSON() fails your dynamic object is broken somehow

You can't just set the dynamic object (exactly the oref) to  %String.
You place than just a reference of a local structure to your message.  That's useless.
you need to 

Set classObj.Test = DynamicObject.%ToJSON()

Now you have your string that gets persisted and is part of your message.

Suggested reading: Converting Dynamic Entities to and from JSON​

I'd recommend JAVA, JDBC Cache, SQL it's a strong and fast combination with strong support,

VB6 is far out of date
VisM is a remainder from before Caché times.

So it's rather an issue of compatibility, maintenance, support  than performance

Objects vs. SQL is depending on what you try to achieve.
Object access is can be an advantage for large and complex objects that you need as a whole item.
As you mention VB6 + VisM the original author probably wasn't very comfortable with SQL at his time.
 

Welcome to the Caché world!

There is NO limit on commands to be executed !
Don't worry about the number. SQL is consuming a lot of commands.
And Class Queries in %SYS by its nature may even consume more than standard SQL queries.
Just during a quick check of your code example I passed 700k.

A general hint on programming style:
Pls. try to avoid Dot Style and use curly brackets instead. 
This is a very old notation. Not as old as Hieroglyphs, but almost smiley

What I mean. instead of

  I Rset.GetData(1) = Pid D
    . w !,Rset.GetData(1)," ",ClientIPAddress," ",Pid
            //Set rs=$SYSTEM.Process.Terminate(Pid)
            //If rs'=1 {
            //    Write "Terminate Fialure "_" Pid "_Pid,!
            //}
            //Else {
            //    W "Sucess",!
        //    }
    //    }

use 

  I Rset.GetData(1) = Pid {
     w !,Rset.GetData(1)," ",ClientIPAddress," ",Pid
            //Set rs=$SYSTEM.Process.Terminate(Pid)
            //If rs'=1 {
            //    Write "Terminate Fialure "_" Pid "_Pid,!
            //}
            //Else {
            //    W "Sucess",!
        //    }
    //    }
    }

 This makes it much easier for the audience to follow your code, and it is actual coding style,
 

Congratulations!

docs say:

 Sends any buffered output out the TCP connection; that is, executes a TCP SEND operation on the data in the output buffer.
If the data is compressed (/GZIP) stream data, *-3 sends the data without marking the compression endpoint.

I interpret this: 
up to some limit, it is sent somehow automatically
and the eventual rest needs to be pushed out manually or gets never sent or is to late.

???  surprise

- you have

Parameter CONNECTION = "canreg,NOCREATE";

- so the name of your Gateway definition is canreg 
- you have no storage for streams defined.
Strange but might be an issue of Caché version version issue.
in 2016.2 it just doesn't compile without.
- more strange is this:

Index MainIndex On null [ IdKeyPrimaryKey ];

The property null doesn't exist in your class.
Without a property null  it doesn't compile for me.

adding manually the missing Storage section and 

Property null As %Integer;

let it compile for me. 

Though this may be a temporary fix I distrust your Link Wizard.
Eventually, you should contact WRC  to investigate the reason.