should I have a char in every group for maximal plot potential (and as an example for each group?)
Honestly depends? I don't know how your groups are set up or how many groups you have. If you have a faction-based plot with roughly 5-6 factions then it's probably a good idea, but don't make characters that you don't actually want to write. If your membergroups are purely based on aesthetics then there's no need, because it's not like having many or few of any group will affect how often those groups can get plots.
I have a preference for shorter lorebooks, but again it's hard to say precisely without knowing what your site is about. (A site with nonhumans will need more lore than a site with only-humans, for example.) I do prefer the option of having a FAQ section in your guidebook so that questions people come up with that weren't already in the lore can be added in on an as-needed basis.
Not strange at all, just like some sites have mods who only do coding and such.
IMO, what kind of archives setup you have is less important than
actually using it. This means proactively archiving your own threads when they're done, having an easy ticket system for members to request their threads be archived, and going through and sweeping out incomplete threads after activity checks where members have left.
You really don't need more than a single archives board, but if you want to subdivide it, I like completed/incompleted/"other" (non-thread things like want ads and plotters) and maybe a board for comms archives if you allow comms threads.
Things I like in a shop:
-character powerups/character slots, if you're a site that gates those things
-tickets to app/promote to site canon positions, if you're a site that includes those roles
-"create your own board/lore" tickets for people who want to add onto the world and have earned the points to show they're a dedicated member
-"member group swap" item to let people sort their ooc accounts automatically. might work on jcink only.
Things that I could honestly take or leave:
-aesthetic badges. If you like 'em they're harmless, if you hate 'em you don't need 'em.
-faceclaim swaps and secondary faceclaims. Not a fan of them but also not gonna get super aggrieved over having it.