I would use Dynamic Arrays and Dynamic Objects to make this simpler. Those two are your best friends when working with JSON.

ClassMethod GetAllPersons() As %Stream.Object
{
    d ..%SetContentType("application/json")
    Set rset = ##class(dc.Sample.Person).ExtentFunc()

    set stream=##class(%Stream.TmpCharacter).%New()
    set dynArray = [].%New()
    While rset.%Next() {
        do ##class(dc.Sample.Person).%OpenId(rset.ID).%JSONExportToString(.myPerson)
        Set dynObj = {}.%FromJSON(myPerson)
        do dynArray.%Push(dynObj)
    }
    d stream.Write(dynArray.%ToJSON())
    return stream
}

I know this has already been answered several ways, but let's not overlook embedded SQL as an option.

&sql(select JSON_OBJECT('Name':Name,'Title':Title,'Company':Company,'Phone':Phone, 'DOB':DOB) INTO :person WHERE ID = 1)
set personobj = {}.%FromJSON(person)

This can get a little unruly for tables with a lot of columns, but if you're wanting to pick out certain specific columns or customize the JSON field names, this approach gives you that flexibility.

It's useful in terminal, and in my opinion, it lets me write neater code too. I don't have to include lines like

if $$$ISERR(sc) { $$$ThrowStatus(sc) }

every time.