Fundamentally it is a technique based skill, which is why most decent field kickers make good goal kickers.
If it were significantly about power then forwards would do the field kicking. Having said that Cameron Smith and a couple of others are an anomaly (classic example of exception proves the rule and also Smith can kick goals which speaks to my earlier point).
It does my head in that New Zealand league players do not come through the juniors spending thousands of hours working on field kicking unlike their Union counterparts. However the tradition of Union in this country actually plays a role as to why we have issues. Most of our kids play both codes, and in Union they learn that kicking out on the full is legitimate end to a phase of play - whereas in League it is the opposite, out on the full is packed with jeopardy for our young players.
I believe this sets up a developmental issue where our play makers are conflicted in their dna. I believe that we have the talent, the issue is the chronic developmental 'mixed' messaging in those formative years, leads to a lack of confidence.
Confidence is so underrated in field kicking. It is well understood in goal kicking (as I already laid out the two are correlated) but it is not talked about much in field kicking.
Shaun Johnson is a really good case study in this phenomena, just before he left the Warriors, he was becoming a long field kicker (there was a season were he was kicking back to back 40/20's) unfortunately the first time SJ put one out on the full he put the 40/20s away (how much of that was our coach telling him to knock it off and kick safe we will never know). Anyway of course SJ went to the Sharks where he was encouraged to work on his organizing skills (field kicking is a subset of organizing play because it comes under game management) and SJ of course went on to become a great field kicker (one can only dream what would have happened had he gone to school in Aus).
For this reason I have always said, if you are parent, and your young league player has aspirations to play in the spine, get them field kicking, because in NZ that skill is so rare that it is a licence to print money.
Adam Reynolds maybe the best field kicker in the game, either him or Nathan Cleary, and they roll Old Reynolds out, limping and hobbling with his arthritic knees, and carry him through games....so in answer to your question more strength is probably not the go.
Can we turn one of ours into a field kicker like SJ did later in his career? I doubt it, because it is something you have to learn young and you have to remember that Shaun Johnson is the most physically gifted player the Warriors have ever seen. That guy had talent and skills which few are blessed with.
Te Maire and Chanel can probably kick long now. But you will find when they do at training the out on the full issue is too high to risk it in a game.