If you haven't already, you should add a reference from Reservation to Account, storing the account that made the reservation as an association. You can then add an XPath in the entity that users can only see their own reservations.
This should be done on domain model level, not page level because otherwise tech savvy users would be able to retrieve other reservations as well, which you want to prevent!
Skip Account, no need for it here.
Upon creating and saving the reservation, associate the reservation to the System.User which is the currentUser. This makes sure that you can retrieve the reservation with xpath constraint [id = $currentUser]
My Reservation entity contains FullName and is referenced to Account and Employee(my company's SSO login entity), my microflow for opening my new reservation page retrieves the account by current user(id is $currentUser) and then changes Reservation.FullName to Account/FullName
So the full name is displayed based on the current user when they go into new reservation page as well as in the active reservation page.
So the saving is ok, the only thing I think is wrong is my logic to show only the reservations that our the current user's
I solved it, for anyone having similar issues, my solution was to implement in my microflow which gets my reservations those retrieve account from database and an xpath constraint [(FullName = $Account_1/FullName)] in retrieve objects from reservation :)