I believe you need to research the Composite String Runners and a handful of other things. Below is a comment from Anrhaa's collection which may be able to help. It might not be exactly 1:1 for this list, but it's mostly the same.

I'll pin this so others can find it in the future.

You need 'Mounted Crossbows' and 'Leather Armour Crafting' to unlock 'Crossbow Turret Strings'

You need 'Crossbow Turret Strings' and 'Crossbow Crafting' to unlock 'Composite Runners'

You need 'Basic Weapon Smithing' to unlock 'Crossbow Bolts' which unlocks 'Mounted Crossbows' and 'Crossbow Crafting'.

Now

'Mounted Crossbows II' requires 'Mounted Crossbows' and 'Crossbow Crafting'.

'Prototype Harpoon Turrets' requires 'Mounted Crossbows II', 'Composite Runners' and 'Crossbow Turret Strings'.

'Harpoon Turrets mkII' requires 'Prototype Harpoon Turrets'.

'Advanced Harpoonery' requires 'Harpoon Turrets mkII'.

and so on...

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2927813323

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3212753028

Yea i figured it out down the line, while I like the complexity of crafting more, Kenshi reasearch UI really isnt great at testing and figuring out what you need for what. Didnt figure for harpoon though, so that is good info.

Some research is present in red, and says what you need for it, while others arent. And with addition of many other crafting items, it is really hard to even see a research that would make sense for certain stuff.

Otherwise, for roleplaying it is an excellent mod, usually I would just build it and be done with it, now I am raiding holy nation mines for that iron and copper, and it is a lot harder to make them if you dont use prospecting, as it should be. Just UI is terrible, and figuring out what you need in a huge list is a bit of a problem.

Haha I know what you mean, I do wish there was a "Rimworld" type research menu as it does get a little more involved with UWE/this collection. Unfortunately it's not really possible to make something like that in game, so the research now is a lot more trial and error to figure out exactly what you need in order to unlock XYZ.

Do note, since you're almost there, you'll need to find/buy the "Outpost IV" research to progress further down the tree (Tier 5+) after you've completed Outpost 1-3. It's not directly obvious in game, so I get a decent amount of questions for it (it's a UWE change).

I suppose the confusing research in itself is somewhat lore friendly, considering the backstory for Kenshi lol. I'm glad you're enjoying the collection! Good luck at the mines!