The LastSubmit property is available in the OnSuccess action. It is the form holding on to the record that it just submitted, so it is instance-specific... so you have 21 people entering records simultaneously, and they are each going to return their own LastSubmit, no matter who wrote in what order.
While I think you can do what you're looking for with the Relate() function, as @v-xida-msft suggested, as a DB admin, I tend to break into hives when I encounter anything that sniffs of entering downstream records before upstream records are established... like entering unrelated line-items and trusting that they will be connected after the fact.
Your relationship between the line items entity and the expense report entity should point to the primary key of the Expense Report entity. That is, ONE Expense Report could have MANY Line Items (1:N). It doesn't matter to the Line Items entity how many times the Expense Report GUID appears in the data, just that EVERY record in the Line Items entity has a valid GUID.
When you create an entity, CDS gives you a default GUID named after the singular version of your item (ie, "ExpenseReport"). That is the field that will drive the relationship to the Line Items entity. That leaves you 2 ways to get or use the GUID...
...either give them a drop-down (that will have all of the fields of the Expense Reports entity) and let the user choose which Expense Report they are updating, or...
...capture the LastSubmit Expense Report (after a successful submission of the form), and write it to a variable that you then use when you write your Line Item entry (like with an invisible control in the Line Item form that reads the GUID into the text property of the control and is then saved with the form contents when the Line Item is committed to CDS).
Bottom line, I think it's simpler than you are making it out to be.