@LaurentG I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue: Event Subscription with System Topic parent for Private Storage Account fails
Solution: In the Storage Account Networking set "Public Network Access" to "Enabled from selected virtual networks and IP addresses" and then choose "Resource instances" and set the "Microsoft.EventGrid/topics" Instance and then the deployment worked successfully!
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.