Basically agree with HGohwell's above post, but generally:
Bios:
I've never personally been much of one to do very in-depth bios tbh. A lot of that information comes out naturally while developing the characters up front, and then after that point they're catalogued and I don't find it particularly helpful to write down their favorite foods or color or zodiac sign or wev, unless something is plot relevant. Basically:
I think some of the approach can change with how many characters are present for a particular project too. Maybe a more in-depth, nitty gritty forum RP style bio works for a story about just a couple of characters, for example a drama or a romance where you're deep diving the protagonist, their closest friend(s), love interest, rival, and maybe a few additional characters.
In a series with... 7 + 6 + 3 +5, 21 prominent characters, you can probably get away with starting with their general characterization and crossing additional bridges as you get there.
Character Arcs:
Now these are important and deserving of some pre-planning, however again I don't think these need to be completely 100% work through and set in stone before starting, especially for a longer project. Of course that couldn't hurt (especially if you're planning to do a full draft before starting which is... a lot of work lol) but generally you can probably get away with defining the beginning, end, and a few key mid points. Connecting those dots can be done as you go and how they naturally fit into the plot. Something like:
A.) Character starts here
B.) failure
C.) realization
D.) development
E.) triumph & growth, end of arc.
There's almost certainly stuff between those points that connects the dots, but as long as you have a clear idea of where they start and end up, I don't think you have to go in-depth on each one... especially since the character arcs should be advancing alongside the main plot anyways.
This also goes back to character quantity- there are so many characters here that there probably won't be a whole lot of room for miscellaneous stuff if you want them all to have arcs and for the plot to not grind to a halt every chapter or two. These developments will need to be very tightly integrated into the plot I think to keep things moving forward, so I would develop the main plot and character arcs together, instead of trying to fully bake a bunch of char arcs only to find that you don't know how to weave them together later.
An example that stands out to me that I've been watching recently is the Digimon Adventure reboot that has been airing this year. This cast faces similar challenges by having 6 kids + 6 digimon partners that all have their own personalities and character arcs. The way that they achieve the needed character development across that many characters while still keeping the plot moving forward... is simply by keeping it really streamlined and hitting a major character development moment every episode right along whatever plot point is being covered that episode. There's not a whole lot of room for extraneous scenes or fluff because there's just so many characters to get through, but they manage it and it's a relatively enjoyable (if not formulaic) experience.