Optimal difficulty to maximize technique improvements?

If you are constantly working out hard-for-you routes or boulders where maybe you start out not being able to do the moves at all and then after some number of attempts learn the moves and then go on to optimize those moves to create bigger and bigger links or send the route, is better movement/technique getting ingrained in you during this process? In other words, do you walk away from that route a fraction of a percent a better climber? 

If you agree with the above premise, where's the sweet spot in terms of difficulty to maximize returns? On a spectrum of even below flash grade--> flash grade--> long term proj.

I feel like the challenge needs to be high enough that you have those opportunties for positive feedback as you adjust but not so hard you aren't regularly being subjected to new/different moves/routes.

...Ultimately, this is all wishful thinking for me because I mostly like doing routes problems I can RP in 5-10 tries or a couple sessions. 

n=1 I like doing routes that seem impossible on the first go and have been doing them for the last 3-4 years, except this year due to injury. Putting down 1-2 hard (for me, 12+) projects a year over 20+ attempts. My technique is still total garbage and I can see it compared to my climbing partners.

I think people who climb a lot in different styles (e.g. lots of stuff in Yosemite and now doing powerful sport stuff) and have lots of mileage including onsight have better technique... problem that I see for myself for projecting is that I can do the move crappy 10 times - if I optimize that one move, most of my repetition is still doing the move poorly and I dunno if this optimization really translates into any situation other than optimizing another move, i.e. thinking on my feet. 

Moreover, when projecting it's possibly to do the move poorly almost every time and then get lucky on one go due to dumb luck (either doing move correctly without realizing why, or botching it and holding on anyway), boom, send :) It actually happened to me just last week where I was trying to figure out how to not have my feet come off on a throw move where my slightly shorter partner's feet never come off; well on one attempt my feet came off but I still held on somehow and now I never need to climb that route again. 

I think you’ll find all the answers and guidance you need from the Anderson brother’s “The Rock Climbers Training Manual”. 

I think you can obtain technical improvements at nearly any grade, provided the route isn't 3 grades above your max RP, if you get on any route, on top rope and be incredibly deliberate with your movement and really feel it out even just working 3-4 move links can be very rewarding and deliver improvements. 

Johnny daws describes a process which he calls "slabification" where he would make a strong active effort to put as much weight through his feet and as little through his hands. I think if this is practiced consistently you will see great gains.

If bouldering is the main discipline, I have heard a lot of people benefit from really studying their movements using their phone camera. 

Most of what I’ve read says the best area for making improvements on routes is in that try hard onsight/flash to 5 tries range.

I think most would agree that milage with a variety of movement styles/rock types will advance your climbing much more than hammering the same route into the ground for a month or two.

Sometimes I see climbers move really well at easier grades and move terribly when it gets harder. Other times, I observe climbers climb sloppily at easier grades and then clean it up for harder grades.  My goal is to be able to use a wide variety of skills in a wide variety of contexts. I systematically track where I do and don't demonstrate a skill. I can then prioritize where I spend my very limited training time. Here is my rubric of climbing skill by context:

My impression is you can make gains from projecting, but mostly in strength and power.  

For me, I can gain strength and power by doing short, intense sessions of limit climbing (1-5 moves), but I have to nail the recovery (quit early, sleep, diet, light cardio, 2-4 days real rest). For technique, I find a lower level of difficulty allows me longer sessions where I can refine movement through repetition and create good habits when I’m climbing tired. True endurance training I do at an even lower level (kind of zone 2-3 cardio). All three overlap, but each has a focus.

As an example, my grades are pretty different between ropes and boulders.  Limit bouldering for me is V7-10, but limit rope climbing is 12b-13a (technically a lower level of difficulty).  If the rope routes are sustained that further drops my limit because I can’t arrive fresh at the hardest moves.  Technique grades for me are V3-5 and mid 5.11.  Endurance grades are V1ish, low 5.10.  

Sorry to spray about myself, but I know my own body experience better than all the different theories.  The hardest things for me are recovery and staying with a schedule.  

Edit:  I’m listing my grades for two reasons.  One as an example that distinctly separates training types by difficulty.  Second to push back on the idea that near-limit climbing is where all the gains are.  It’s a trap to always climb near your limit and it slows progression.  That means putting your ego away and climbing lower grades that you can’t brag about.  I struggle with this and I think a lot of others do as well.

I think it depends on what you mean by technique improvement.

To learn a completely new complex skill, you need to first try it on a REALLY easy terrain. Once you understand the mechanics of the movement, you try to deliberately apply it in a slightly harder situation. You look for a chance to use that specific technique, even though the route/problem is easy enough for you to do the move held a dozen different ways.

Usually when the climbing gets hard enough, we default to our strengths, and relatively-new skills don’t come to the front of your mind. And if sending is the goal, that’s understandable. So you have to first force yourself to use the new technique in training scenarios, so the desire to send doesn’t fight with the desire to improve that particular facet of skills.

But working problems at your limit ABSOLUTELY improves your climbing, too. And if rope climbing is your goal, then both limit bouldering, and projecting hard routes are going to be beneficial.

These things are not mutually exclusive. But it all takes time. A lot of time. Months and years  not weeks  

You definitely build technique trying limit routes, but I think that's only the most efficient path when your technique is already pretty refined in a given area. Otherwise, mileage probably makes more sense.

For example, I feel reasonably good about my indoor board technique, such that doing a bunch at my flash level doesn't feel maximally helpful (certainly not to say there's nothing to learn), but I feel like I'm still learning lots of tiny technique/tactics adjustments when projecting at my limit. On the other hand, my offwidth technique isn't all that refined, and I think I need to build my general OW intuition and movement library before it makes sense to optimize beta for limit climbs. To send a single OW at my limit, I could fiddle around and memorize the exact moves and execute them, but my overall technique probably wouldn't improve much because my learning would be over-fit to a particular route and I wouldn't understand that type of movement well enough to generalize it. So to get better, I'd probably prioritize mileage.

With that in mind, I think you need to think about a base you have in the technique you want to progress in. For myself, I feel like OW, friction slab, frictiony stemming, and comp-style dynos are all areas where I have little base and I'd prioritize easy mileage, while most sorts of gym and granite bouldering (except mantels and dynos...) feel like limit projecting is appropriate for my learning. Other areas of technique are someplace in between and maybe suitable for flash-level work.