Yesssss, public outcry for Saxon elective, this is what we need.
Get me ten more pages of discussion and perhaps I can wrangle Code into making it for me. Swarm, fellow Anglo-Saxon fans, swarm!
Could modifying the Witan tradition of the Anglo Saxons be the key to this problem? Like, maybe underage kids don't inherit if their parent has living adult brothers and/or sons?
FWIW, regrettably, succession between titles like this is all hardcoded, none of it's exposed to script. That means that the only way to create or adjust succession types like this is to be a programmer on the project and make them manually - as I understand it, not the hardest thing, but not the easiest either. The key problem (AFAIK - and I am a humble pleb designer) isn't actually setting the thing up, it's performance, it's accounting for edge cases, it's making it work in such a way that it doesn't bork the title if used by hundreds or thousands of characters.
Problem is perhaps over-egging it, that's just sorta... an obstacle in development. Every part of development has obstacles, or else we'd just will things into existence. Unfortunately, because we've not focused on content for either the British Isles or the Rus', we've not had official time to sort this succession type, so it's been sat on the back-burner since release. If it was scriptable, I'd have scripted it already.
The dream'd be having all succession types exposed to script - regent succession actually is and I had a blast composing the factors that determine who's top of the pile there - but I'm pretty certain performance'd just choke on its own tongue and die wheezing if we did that. Anything scriptable is always slower and more performance-intensive than the same in code. Anything called frequently, such as who your heir is, or involving lots of entities, such as every title you hold, or lots of characters, such as every landed ruler on the map plus a few dozen extras for the landless rulers, gets fractally slower and slower. Whiiiiich means any new succession that
isn't elective costs coder time, and Code's time is usually at a bit of a premium since they need to make most of the actual features.
The solution seems like it should be pretty simple to me. Saxon succession, witan succession, however you want to call it should be a form of elective monarchy where the eligible candidates are the current ruler's family, claimants to the throne, and any vassals over the rank of duke. Opinion should be weighted more heavily towards the elder members of the current dynasty.

Good news for ye, because that already exists in the title, more or less, and it's called Saxon Elective. It unfortunately isn't really the solution here. Electives don't work very well at the duchy level, and they're per title - it'd
kinda work, but it wouldn't really be less hacky than the current solution or the alternative proposed solutions, it'd just be hacky in different ways. We'll get rota succession/sibling succession/whatever we want to call it some day, if I have to spit blood and teeth over this forever, buuut it's gonna require either assigned code time, judicious use of bribery, or substantial finegling, and it's not the only thing on the table requiring any/all of those.