There may be a better way, but you could do the following:

• Create tracking tokens like MostRecentID and MostRecentSubmission
• Use a For Each Value activity on submission timestamps
• Use a conditional activity on the current value to see if MostRecentSubmission is empty or less than the current value. If so, replace MostRecentSubmission with the current value and replace MostRecentID with the value at the current iteration in Submission IDs.

With this method, at the end of the For Each Value loop, you will have MostRecentID with the most recent Submission ID and MostRecentSubmission with the most recent submission timestamp.