The 2020 graffity likely means nothing. It might not even mean the year 2020, also if someone graffites the number 2020 in a wall during the year 2020, that wall will have that number graffited for the years that follow. At best this means this is set in or after 2020.
However, I don't see no direct indication to suggest Shang-Chi HAS to be placed after Endgame (other than it will be released after Avengers: Endgame, obviously). If it is set in 2020, that would actually be great. I think that Shang-Chi would be like this humble guy that eventually looses the people he loves during the snap, and that drives him to those paths that eventually create his heroic character. Eventually a sequel would be set after Endgame and could be focusing on him trying to find the person he cared about that was returned from the dead. Whose person is this? Maybe his girlfriend, or fiancée, maybe his adoptive father or mother, or even real mother, assuming his father will be The Mandarin. I think this could be important to define Shang-Chi as an interesting character, dislocated from a world he wasn't able to adapt to, and as the sequels progressed, he will increasingly adapt to the changes, value the people he loves and become an important person in its greater world (i.e., interact with other major MCU characters). I think this could be an important character development for this character, one that would also benefit from the five year gap setting.
Also, someone mentioned earlier than The Falcon and The Winter Soldier will be set in the fall of 2023. I personally don't think it will be that soon. Given that Endgame's events already happen two or three months before the beginning of the year 2024, and because I'm expecting a "year later" treatment, so that people will more easily adapt themselves after the traumatic events that followed, I believe The Falcon and The Winter Soldier will be set in fall of 2024, instead. Could be in the fall of 2023 too, as we see in the trailer Falcon training with Cap's shield, as if he didn't mastered it yet, also pointing that maybe not too much time happened since Endgame. Still, I think that making all that plot happen just a month or a month and a half after Endgame seems a bit too soon. So I lean to a late 2024 date, if the series is indeed set in the fall.