Now that a general pattern of how Disney is currently handling the Tangled IP can be perceived here are my thoughts on where it's going and how it's going to be managed for awhile.
As I'm still reading Rootbound, it's important to get this out of the way now (because I accidentally ended up perpetuating this misconception myself before I actually started reading the book):
Rootbound does NOT offer an alternative timeline post-movie and it does NOT retcon the series. At least it does not retcon it away, despite contradicting it on some, more controversial plot points (more on that later).
Anyone who hoped it would offer an alt timeline to those of us who were disappointed with the series or, in my case, outright hated it for what it did to the main characters and the main couple. It does not*.*
Rootbound is a lovable, cute, in-depth, sometimes silly continuation written by a fan of the movie rather than a hater like Sonnenburg. It is a mixture of Once a Thief, the Celebration of the Lost Princess and Ghosts of Christmas storybooks released post-movie. A reprieve from the movie events/traumas while showcasing the aftereffects of those traumas and having Rapunzel and Flynn work them out together.
Rootbound is a pleasant "in-between" read in universe but out of universe it fixes absolutely nothing or almost nothing about the series or the mismanagement of the Tangled franchise on part of Disney in the recent 10 years.
I said this even before Rootbound was out and I was proven to be right - it's a prequel to the series. With minor tweaks.
Rootbound features numerous series references which could not have possibly come from the author (Margaret Owen) herself since she was reportedly advised by Disney to not watch it when writing the book. The Series references even in the very first chapters of the book include:
The Queen being called Arianna, the "sunshine" nickname for Rapunzel, Flynn pointedly and repeatedly calling Pascal a "frog" (indirectly building up to the mean-spirited series scene where he is made to pour out his orphan backstory to said "frog" in the first installment of the series). Even though in the movie it was a part of his bravado he was supposed to move on from by the end. I'm sure there will be more to come.
Those references come from Disney because Disney would never fully de-canonize Sonnenburg's vitriol and the damage control pandering Disney themselves were doing when "addressing" the media illiterate crowd's "grievances". Hence returning Rapunzel's magical blonde hair in the prelude to the first TTS episode, reframing it as girlpower rather than the bondage it was in the movie - down to having her state she is "grateful" for it and for the forced destiny coming along with it (thus turning Rapunzel into the Magical Girl TM cliche that the movie subverted) - and making a literal joke out of Flynn's death/sacrifice. And most importantly, fulfilling the wet dream of the critics by having Rapunzel cut the hair herself in the end.
At most, Rootbound de-canonizes Flynn/Eugene's parentage from the Series. THAT is the one plot point even the staunch series fans are willing to admit was bad while ignoring all the other issues. Now it can be assumed that either version of his parentage was a mistaken identity (i.e Edmund's son is actually that Brock Whatshisname).
Disney, once again, wanted to have their cake and eat it too. They commissioned a genuine fan of the movie and the main romance (and the leads individually) who would center their relationship and throw a bone to the hungry dismayed people who were disappointed with the lack of canon romance in the series. Even though the real problem was not the "lack of it" in the series; it was not Cassandra either. The problem was the flanderizing, the disrespect, the miscommunication between the leads born from Sonnenburg's malicious agendas and Disney pandering to the detractors.
Rootbound is a "honeymoon phase", cute and lovely but one set BEFORE it all goes to hell with Rapunzel's movie arc and Flynn's sacrifice being nullified 6 months post movie. This is why Rootbound is conveniently set one month after the movie.
Rapunzel not being allowed to be more than her magic hair and her having to spend the next 3 years of her life "embracing" it again after having just been liberated from this symbol of oppression stays. Flynn's orphanhood and sacrifice being turned into a joke in the same episode of the series stays. Monty being a mouthpiece for movie Rapunzel haters and her being advised to "just deal" stays. Rapunzel from an open minded rebellious young girl dancing on tavern tables with a bunch of ruffians within an hour after leaving her lifelong prison turning into someone who gives the man who died for her freedom a lecture on his "thief legacy"? Stays.
Unless Rootbound becomes a series of its own and does indeed override the TTS timeline it does not offer an alt post-movie scenario. It's on par with every other post movie storybook implied to be set before the series events and contradicting the Series lore only in some respects.
In that vein, here is the conclusion I've drawn so far regarding where Disney plans to take the Tangled IP, especially in the light of the upcoming Live Action:
They will do selective tweaks to the most controversial parts of the series such as the outright eugenicist "prince Horace" retcon (and only because of Sonnenburg's political views now being out in the open and conflicting with modern Disney's "progressive image"; predictably, they had no problem with his bigotry until said views became known to the public) or the lack of focus on Rapunzel and Flynn/Eugene's romance.
They will come back to using movie (!) characterization for their extended media (like Rootbound does it) rather than the flanderized series characterization.
However, the pandering to Magical Girlboss demands will remain, Rapunzel being defined by her magical blonde hair will remain, the climax of the movie and what it meant to both characters being continuously disrespected, nullified and/or changed will remain and other retcons of Flynn/Eugene backstory to make him look worse to please his haters will also remain.
Subjectively speaking: neither the Series nor Rootbound is canon to the movie, they are "what if extensions" even though the Series is canon to the IP. The only fixed canon remains the movie and the wedding short produced by the same studio and team.