Like you mentioned, computed columns will not calculate until the form is submitted. I think you would have to create a custom form to get what you want.
I’m just dropping this here so I have something to refer people to when they ask.
This is a simple concept app (copyable), that demonstrates the following techniques:
Building custom forms for adding and editing list items (as an alternative to the native forms provided by Glide)
Preventing duplicate new entries using User Specific Columns and Relations
Enforcing mandatory input items
The use of visibility conditions and user specific booleans to control user flow
There is nothing particula…