Would this work for you?
The original date is in cell A3, the returned date is on row 4.
=LET(
StartDate, $A$3,
EndPoint, B2,
EndMonth, EDATE(StartDate, EndPoint),
MonthFraction, EndPoint - INT(EndPoint),
DaysToAdd, INT(DAY(EOMONTH(EndMonth, 0)) * MonthFraction),
Result, SUM(EndMonth, DaysToAdd),
Result
)