hi @Juliet it look you have two conditions
1. evaluate the choice column
2. check the selected date and get first available static date
as your title is find date I will give a solution to number 2, if you need assistance with number let me know
you will need the static dates in a collection, a datepicker and a label to show the first available date
Collect the static dates, lest call it colSdates, in the example i added a few dates for illustration below
ClearCollect(colSdates,["01 August 2021", "22 August 2021", "15 November 2021"])
and then i added it to a gallery

Now lets add the datepicker which in you rcase will be the "Planned submit date", lets call it dtPlannedDate. add a label to illustrate the first available date lblSubmitDate
Set the label.text to
First(
Filter(colSdates,
DateDiff(DateValue(Value,"en-US"), dtPlannedDate.SelectedDate)<=0
)
).Value
the solution will be as follows:
when the use selects the 1 Aug as planned submit date the response will be

now lets select 2 aug and we expect the response to be the 22 aug right

Lets do the 15th and 16th nov

16th epty as no static dates are defined

hope this helps