You can use the STUFF() method to insert characters into your string to format it in to a value SQL Server will be able to understand:

DECLARE @datestring NVARCHAR(20) = '20120225143620'

-- desired format: '20120225 14:36:20'
SET @datestring = STUFF(STUFF(STUFF(@datestring,13,0,':'),11,0,':'),9,0,' ')

SELECT CONVERT(DATETIME, @datestring) AS FormattedDate

Output:

FormattedDate
=======================
2012-02-25 14:36:20.000

This approach will work if your string is always the same length and format, and it works from the end of the string to the start to produce a value in this format: YYYYMMDD HH:MM:SS

For this, you don't need to separate the date portion in anyway, as SQL Server will be able to understand it as it's formatted.

Related Reading:

STUFF (Transact-SQL)
The STUFF function inserts a string into another string. It deletes a specified length of characters in the first string at the start position and then inserts the second string into the first string at the start position.

STUFF ( character_expression , start , length , replaceWith_expression )

DECLARE @datestring NVARCHAR(20) = '20220822190000330';
DECLARE @dateint BIGINT = CONVERT(BIGINT,@datestring);
SELECT CONVERT(DATETIME,FORMAT(@dateint, '####-##-## ##:##:##:###')) AS FormatDate;

Output

FormatDate
-----------------------
2022-08-22 19:00:00.330

SELECT format(getdate(),'yyyyMMddHHmmssffff')

Please run the below sql command for unique key generation with timestamp