Tech-Trees are best used to create dynamism, counterplay, and replayability. Using these three pillars I would hunker down on each one and sketch out how you want each to work in King's and Knights.
To explain a little bit of what I mean by each:
Dynamism: The ability to change the playing field between these four kings. Ostensibly a player shouldn't ever be certain what another king is committing to next in the early game. It even gives some incentive to scouting, if I see early investments in archers that should mean I feel pressure to adapt.
Counterplay: Basic action into reaction gameplay. If a Ranger knight is slaughtering me out in the field then there should be a accessible response in my tech tree as a king. Perhaps buried under a tier or so of research but never behind unrelated upgrades. I as a player should not feel like I lost because I picked the wrong tech branch 10 minutes ago as an example.
Replayability: Each path should feel 'viable enough' that players don't just fall into the same build every game. It does not have to be the epitomy of balance but it should feel like there is some level of 'choice'.
So if you're looking for some inspiration I would actually point to the early World of Warcraft classic talent trees. Don't get me wrong they were far from perfectly designed (they were very flawed lol) but they exceled because of how they 'felt' to most players; You commit to a column (your specialization such as Arms, or Frost, or Holy, etc.) which gave you early identity but could then still dip your toes into other paths for utility or expression. I think this is EXACTLY what you want for Kings.
So I would reframe the scope first as I (personally) do not think that the tree is "too big" I just think it could benefit from some clearer lanes. Keep it as an agnostic Human core but maybe frame some focused columns like these talent trees. If you want some extra references for the kings I would look at some of the custom melee races. Alot of folks put some insane time and talent into making these cool factions and you can mix and match them on the basis of research. I think this would also do a lot of good for the King players as I recall King players often falling into the role simply because Knights were not available or because the game would not ensue without someone assuming the mantle. To this end if a king player is joining the lobby to have something akin to a soft-rpg experience similar to the knights then having the tech-tree mimic such early rpg talent progression surely could not hurt.
Hopefully some part of my little word vomit here helps. I understand this isn't exactly a direct answer to your question but I think it is a VERY important thing to keep in mind while you ponder the scope of the tech trees. Keep on rockin fren.
