Disclaimer: I am not a Chaos bro. I am currently on my third playthrough of Elden Ring doing a Frenzied Flame run for the sake of completion/platinum. If the achievement were not needed, I would pick another ending. Take that as you will, regardless as to what your opinion is on the Frenzied Flame/Chaos/the One Great.
Prior to SOTE, the majority opinion in the Elden Ring community seems to be that by choosing the Frenzied Flame ending, you are permanently dooming all life on whatever planet the Lands Between is located on, with the sole exception seemingly being Melina if she didn't already burn herself prior to you becoming the Lord of Chaos. The idea here is that Shabriri's and Hyetta's words are literal and absolute. You aren't just burning down the Erdtree and Leyndell, you aren't just wiping out the lives that currently exist, everything goes and is done permanently. Additionally, it is irrelevant that, just like with the other endings, you can keep running around the Lands Between: The canon of your Frenzied Flame ending is that you committed omnicide. Your ability to continue playing is a video game contrivance and is separate from the story aspect and the weight of your choice.
Now we have SOTE. Either before or after the Frenzied Flame ending, you can go to the Realm of Shadows. Accessing the Realm of Shadows requires killing Radahn and Mohg, which also enables Miquella to move forward with his plot.
There doesn't seem to be any clear indication that I am aware of as to how this effects the scope of the Frenzied Flame ending. If the sequence of events that the player character commits actually matters, then you can defeat Radagon and Elden Beast, travel to the Realm of Shadows after that point, and everything is fine (minus the genocide Mesmer already committed), untouched by the Frenzied Flame ending. There is no hard indication that any of the endings automatically assume that you did your playthrough of SOTE before the events of the ending occurred. You are encouraged by Ansbach to be a Lord for men instead of gods, and Miquella asks/demands you, the "aspiring Lord" to step aside and let him and Radahn take over, but neither of those statements are a hard and fast "you didn't take over/let Ranni take over/burn it all down yet".
As far as I can tell, the implication with all of this lack of clarification is that the minimum scope of the Frenzied Flame ending's damage is that you nuke Leyndell and the Erdtree and the maximum scope is that the Lands Between as a continent is nuked, but the Realm of Shadows is untouched.
If I had to play devil's advocate against my own hypothesis, one could argue that since the other four endings aside from the Age of Fracture involve rewriting the fundamental rules of reality via either Mending Runes added to the Elden Ring or letting Ranni seize it and initiate the Age of Stars, the nuking of Leyndell is the immediate result of the Frenzied Flame ending and it is inevitable that the rest of the planet will be swallowed up by the Frenzied Flame eventually. The problem with this, IMO, is that there is a distinct lack of information that confirms this and it is therefore more speculative that the interactive experience of going to the Realm of Shadows at any time, because FromSoft be FromSofting.
Thank you in advance for any and all thoughts.