I am very much aware of this question from 2018 by @uhoh - Why would InSight's arrival date at Mars be fixed, and independent of the launch date?. I was led here by a similar question which arose in my mind after reading this article which says -
No matter what day Perseverance lifts off during its July 17-Aug. 11 launch period, it will land at Mars' Jezero Crater on Feb. 18, 2021.
I am not satisfied with the answers to the earlier question which mainly talks about a couple of reasons -
'Operations planning' referring to the sequence of sending data for EDL and afterward by MRO and Mars Odyssey orbiter. I checked for the orbital period of these two spacecraft and they are 35.5 hrs and 2 hrs respectively. So, whichever configuration you start with, they will attain the same configuration in 71 hrs [LCM(35.5, 2)]. That doesn't justify the fixed landing date independent of 26 days launch window.
About the launch vehicle excess energy ($C_{3}$) which, considering the respective payload mass (InSight [ $\approx$700kg ] and Mars 2020 [ $\approx$1100 kg]), have C3 value of around 60$Km^{2}/s^{2}$. See my Payload Mass vs. $C_{3}$ plot for Atlas V in 401 and 541 configurations here made using NASA's Launch Vehicle Performance website. The answer then says that since we have far more energy than requires for Mars mission ($C_{3} = 12 Km^{2}/s^{2}$; from Wikipedia), hence it allows us to select the landing date with high precision. How? The discussion then leads to landing date being chosen so as to fall between Thanksgiving and Christmas but I don't think such a PR case can be made for February 2021.
I am looking for a comprehensible explanation as to why the landing date is independent of the launch date (within the window) for the Mars 2020 mission.