Parmandur
Book-Friend, he/him
320 pages for the DM book would put it at the same length as the PHB or DMG, so that makes sense. Your breakdown of the player book makes sense at around 160 pages, as the successor of SCAG.I think 8 subclasses, even with illustrations will take up 8 to 12 pages max, not 20, only Wild Magic Sorcerer takes up 2 pages never mind 2+ pages.
I think species section would probably max out at 1 page each except for maybe Humans, Dwarves, and Elves. Still it they have pages for say Gobliniods, Genasi, and a some other major FR species mentioned in Monsters of the Multiverse as well. So it could be shorter or longer depending if that is PHB Species only or MotM as well at all. Although if it's only setting lore for each species, it could be part of the 50 general lore pages as well. Hardest one to guess.
Feats really don't take up alot of room, nor do backgrounds, so 40 pages is really the high end, probably more like 30 pages, that's still room enough for a ton of backgrounds and feats.
I agree with on spells, except I think Circle Casting will be included in that 30 pages and outside of more basuc circle casting rules & universal alternations that can be applied to PHB and other spells, will mechanically be mostly contained with the books Spells themselves, So say Doomtide may have as part of the spell itself unique circle casting mechanic. That could push that up to 40 pages, but probably 30 pages like you said.
The one thing you may not know about, but which I heard about on reddit, is the Faction Section uses Faction Renown to grant new Bastion Options. That could take a fair size chunk of the book.
If I'm right and the species thing is part of the 50 pages of lore, then we have say 30 pages for feats and Backgrounds, 30 to 40 ,pages for spells/circle casting, 12 pages for Subclasses. Add a few more pages for odds & ends and you have roughly the same page count as the SCAG. 159 pages.
That would leave the rest of 480 pages to whatever they are calling the other book, I forget, 321 pages, about the size of Waterdeep: Dungeon of the Mad Mage, which is a big ass book, Undermountain basically being a big highly detailed mini setting in its own right as well as an adventure.
So, out of a theoretical 320 pages, we know that 50 pages are for the 50 low prep Adventures, there are 5 key mini-Setting which are likely to be no less than 10-12 pages each (maybe significantly more), assume about 60-70 pages for fhe Bestiary...