According to TheOneRing's sources, the series has officially licensed material from the Tolkien Estate for J.R.R. Tolkien's The Silmarillion (1977) and Unfinished Tales (1980), posthumous publications which include the bulk of Tolkien's detailed notes and writings on the Second Age of Middle-earth's history. This is the first time the Tolkien Estate has licensed new material to film-makers; previous film versions of The Hobbit and The Lord of the Rings descend from a 1969 deal between Tolkien himself and an American producer.
The Tolkien Estate has three lore experts/Tolkien scholars assisting the project and are so far impressed at how things are going.
On the production side of things, scripts are only available digitally and (we know from other reports) are tailored to what scenes actors are in; most actors don't know the full story for the series since they only received scripts for their specific scenes. Presumably the regular actors might have a better idea of the overall story arc, or at least their part of it.
The first two episodes, which were shot months ahead of the rest of the production, have been produced as a stand-alone entry point to the series and franchise. This may (or may not be) fuel rumours that these two episodes will be released separately prior to the rest of the series as a stand-alone, feature-length production to whet the appetite ahead of the rest of the season. However, with only eight episodes in total in Season 1, it's unclear if Amazon would want to split an already short season rather than keeping a lot of discussion going on for longer.
The show has divided production between three units, each dedicated to one of the major races involved in the storyline: humans, elves and dwarves. This makes it sound like each race will have at least one POV in the ongoing storyline. There is also a specific "spoiler unit" which has shot fake scenes to throw off reporters.
Main unit shooting effectively wrapped in April 2021. We know from other sources that a formal wrap on Season 1 is not expected until 30 July, so it sounds like they're doing pickups, reshoots and plates at the moment. There are conflicting reports on when Season 2 will start shooting, some saying January 2022, others earlier.
Lord of the Rings and Wheel of Time are being shot on opposite sides of the globe, but they are pooling some crew and experience: we know that Wayne Yip has directed episodes of both shows and that former Game of Thrones writer Bryan Cogman advised on both series before spooling up his own original project for Amazon.
The show is aiming for a "mid-2022" release date, rather than the early to spring 2022 date previously rumoured.
In terms of the story, it sounds like the show will indeed be delving into early-mid Second Age and will focus on the great elven smith of Celebrimbor as a key character. Celebrimbor (inadvertently) helps Sauron, disguised as a fair prince named Annatar, begin forging the Rings of Power before realising his mistakes. The wording is imprecise, but it sounds like either Sauron/Annatar will not appear in Season 1, or he will but his true identity will not be revealed. That also seemingly confirms that the forging of the One Ring will not take place in Season 1.
Nudity is reportedly present in the show in small amounts, but no sex. One reported scene is a flashback to the First Age showing how elves were captured, imprisoned in terrible conditions and then corrupted into becoming orcs, or their ancestors.
Elves will reportedly have shorter hair than in the chronologically later movies. It's unclear if this applies just to one subset of elves - the Noldor play a big role in the story when they are largely absent from the later time periods - or to all of them. It's also unclear if this applies to the characters who do have longer hair in the chronologically later films, like Elrond and Galadriel, who are expected to play a role in this series.
In probably the biggest spoiler from the report, the show will feature hobbits. In Tolkien's source material, hobbits are not really mentioned prior to the Third Age, with the events of the War of the Ring taking place some 5,000 years after the events of the TV series and the Shire only being founded 1,400 years or so earlier. However, the material also suggests that hobbits were present in Middle-earth earlier, living in other parts of the continent. The show will apparently feature one tribe of proto-hobbits, with Sir Lenny Henry's character being one of this group. However, since they're not getting the specific production unit treatment of the humans, elves and dwarves, it sounds like these hobbits will not play a key role in the narrative.
In a slightly confusing point, apparently the show cannot use the term "cave troll" due to licensing restrictions, and instead will use the term "ice troll." However, Amazon have licensed the existing screen rights for The Lord of the Rings from Warner Brothers/New Line, so it is unclear why they would not be able to use terminology from those films or the novels, especially given all the other terminology you assume they're going to be using/reusing (Nazgul, Sauron, Gondor etc).
Take this all with some caution at the moment, but the reports are broadly in line with previous reports and rumours, and confirm a huge production on a massive scale (albeit one that could probably do with some tighter safety restrictions).
More news when it becomes available.