I may look into that, but what is so bad about the value you get from being able to access her early on, alongside with her starting units?

There isn't anything bad per say, and it's nice to start with her so she can level up. All the base game LL's start with some bonus though, so that's why I'd like to see her get one too. It's hard to pass up Gelt's armor bonus too.

Well, all of them start with unique bonuses and many of them are actually character / lord army based, though I'm not sure if there is another vanilla one that has no factionwide bonus at all.

All of the vanilla legendary lords have two parts: one or more Lord Effects (from faction selection) and a Unique Trait (only visible in-game). As far as I know the Lord Effects only get applied to a campaign if they are selected as your main leader, while the Unique Trait is always there. Mods don't have to follow this "rule", but the vanilla game does (and it'd be nice to see more mods follow that "rule" too tbh).

Notably some Lord Effects are specific to that lord's army (like Wurrzag's -50% chance of successful enemy agent actions against his army), but only appear if they are selected as your leader (i.e. confederating Wurrzag won't include that -50% chance AFAIK).

I'd just like to see her get some Lord Effects as well outside of her Unique trait (which she'll always have reguardless of who is the main leader), even if they only affect herself/her army.

Really, what you see as "rule" is more of a generic and recurring design decision CA took with most, but not all lords. Lord and faction effects are quite variable and generally geared towards making the campaign easy for the player, with some powerful effects, like Vlad's Army-wide Vanguard Deployment or Ghorst's Army-wide Poison Attacks being only active if played by a human, although those are luckily exceptions.

Modding is about breaking a lot of these "rules" and being innovative, introducing new stuff and properly balancing while using what CA provides as a baseline and I'm especially a fan of making things a bit more challenging and balanced - no matter if a faction is played by a human or the AI.
That is why I added for example those two effects to these LLs, regardless of who plays them.

Meanwhile, some core effects that are displayed as lord effects are really just replacing the lord's single trait, like for example Durthu's Army-wide attrition immunity, so that is not so unusual.

Due to the fact that basically all of her special effects are associated to her presence, that makes her even more valuable as a secondary lord, as you really lose nothing by getting her later on, just like starting with Isabella and then Vlad later on.

How do you recruit Elspeth? I'm playing Wissenland as Gelt. There is no tooltip.

Build a Mage Tower, just like for Gelt.