Hi Tez,
After user clicked "I confirm" button of DOI email, his consent level field will be changed, or he will be added/removed from specific subscription list.
Therefore, you could run 2 flows to send email 2:
Flow 1: Trigger it on update of Contact entity and when consent level field of contact is changed, then perform "send email" action.
Flow 2: Please refer to the following tutorial to set up the second flow.
https://crmtipoftheday.com/1362/trigger-power-automate-on-associate-disassociate/
But it will always send email after field changing or contact joining/quitting a subscription list.
If you are using marketing page as "Thank you" page, so another option is to create a segment to query contacts who visited the "Thank you" page and select a specific customer journey as filter.

Because after recipient clicked the "I confirm" button, he will be redirected to the "Thank you" page.

Finally, run another customer journey for the segment to send email 2, the email will be only sent one time.
Regards,
Clofly