Delay calculated control execution until a single line text field is populated

Hi, I am using Nintex for SharePoint 2019 on premises using a responsive form.

I need to calculate and display a custom format registration number. Currently I do so via workflow and it works fine except that the registration number is not visible to the user in the form until the "Save and continue" button is clicked, wait 10secs or so then refresh the form. This is not a great user experience.

I am looking for a way to calculate this registration number (without a workflow, i.e. inside the form) once the "unit" single line text control is populated with data. Using a calculated control in the form doesn't work because the value for the calculated control is "calculated" at the time of form opening and the unit field has not yet been populated. So the result is missing this unit data (the other parts of the number are fine). Responsive forms don't support javascript anymore and there is no inline function "wait" or similar (like its workflow action). Refreshing the form after unit entry before saving it doesn't work either.

Any ideas as to how to make this work?

Thanks