In reply to OG:
> a sort of half crimp with the finger tips flipped down rather than horizontal (not sure if this has a Name).
This is called crimping.
> is it always going to be weak, given the lack skeletal support in half crimp with the double jointed fingertip.
No, weakness is weak muscles, which can be trained, not the lack of skeletal support
>Or is everyone’s fingers like this?
Everyone is made slightly different. For instance I have short pinkies (tip only just reaches halfway up the second joint of my middle 3) which makes a 4 finger half crimp quite awkward and significantly weaker than my middle 3 alone.
>Or do I just need to practice.
Practice training to get stronger
> I am having to do max hangs with half crimp at -18 (yes, that’s 18kg taken off with a pulley). I climb up to 6b and V5 indoors, 6a and VS on the odd occasion I get outdoors, and everyone else at those grades seems to be able to at least do bodyweight hangs.
Great news, this means that you have better technique than your mates and will climb much harder if you can improve your finger strength.
> Is it worth training this half crimp, wondering if either this is something to keep training in the hope it gets better, as a potential blocker for harder routes,
Yes, if LatticeTraining are to be believed then training in half crimp will improve your strength in open and full-crimp
> open hand where I might actually be able to get strong (rather than merely not super weak).
If you want to climb well on pockets you have to have good open grip strength, because there isn't room to get the fingers in that crimp position in a pocket. Also worth noting that open hand is more reliant on friction, especially between the crease of you finger joint and the edge of the hold unless it has a very large radius, fooling you into thinking you are stronger than you actually are.
> or sack it off and focus on full crimp
With a sack it off attitude you'll never get strong on anything, but if you only want to climb on very small holds, focus on training that grip.