Multiple string replacements
A SQLCLR function could be used to emulate the TRANSLATE Transact-SQL function new to SQL Server 2017.

Function definition

CREATE ASSEMBLY [Translate] AUTHORIZATION [dbo]
FROM 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WITH PERMISSION_SET = SAFE;
GO
CREATE FUNCTION [dbo].[Translate]
(
    @Input nvarchar(4000), 
    @Find nvarchar(4000), 
    @Replace nvarchar(4000)
)
RETURNS nvarchar(4000)
AS EXTERNAL NAME 
    [Translate].[UserDefinedFunctions].[Translate];


Usage

SELECT
    S.[name],
    S.string1,
    Result = dbo.Translate(S.string1, N'ACD', N'594')
FROM #String1 AS S;


╔════════╦═════════╦════════╗
║  name  ║ string1 ║ Result ║
╠════════╬═════════╬════════╣
║ John   ║ AB      ║ 5B     ║
║ Kyle   ║ ABC     ║ 5B9    ║
║ Steven ║ ABCD    ║ 5B94   ║
╚════════╩═════════╩════════╝


This simple demo implementation uses a case-sensitive comparison.

Source code

using Microsoft.SqlServer.Server;
using System;
using System.Data.SqlTypes;

public partial class UserDefinedFunctions
{
    [SqlFunction(
        DataAccess = DataAccessKind.None,
        IsDeterministic = true,
        IsPrecise = true,
        SystemDataAccess = SystemDataAccessKind.None
    )]
    [return: SqlFacet(IsFixedLength = false, IsNullable = false, MaxSize = 4000)]
    public static SqlChars Translate
        (
            [SqlFacet(IsFixedLength = false, IsNullable = false, MaxSize = 4000)]
            SqlChars Input,
            [SqlFacet(IsFixedLength = false, IsNullable = false, MaxSize = 4000)]
            SqlChars Find,
            [SqlFacet(IsFixedLength = false, IsNullable = false, MaxSize = 4000)]
            SqlChars Replace
        )
    {
        if (Input.IsNull || Find.IsNull || Replace.IsNull)
        {
            // Return unchanged input for any NULL parameters
            return Input;
        }

        if (Find.Length != Replace.Length)
        {
            throw new ArgumentException("Find and Replace parameters must have the same length.");
        }

        // For each character in the input string
        for (int i = 0; i &lt; Input.Length; i++)
        {
            // For each character in the Find string
            for (int j = 0; j &lt; Find.Length; j++)
            {
                // If the character matches...
                if (Input[i] == Find[j])
                {
                    // ...replace it
                    Input[i] = Replace[j];
                }
            }
        }
        return Input;
    }
}