Makaton is a sign language that is specifically made to fit this sort of niche. We only used a few words: "more", "drink" and "food" being the main three - though our friend has got a lot of value out of "nappy change".
This is not my research area but this list looks really relevant. Thanks for posting it!
For those that do not know. Survey methods and survey analysis is a field of academic research in itself. There are people who specialise in this topic - and hence we can learn from them or pay them to consult on the design of our surveys.
E.g. SMAG (survey methods and analysis group) at the university of Manchester. And NCRM (national centre for research methods) are two I know if in the UK.
There is a "journal of survey statistics and methodology" and "of social research methodology".
And undergraduate textbooks such as
There is a related story of a climber who set out to climb the highest peak in each continent. He succeeded and has some incredible successes and stories along the way but the thing I took from it was that the book is an incredible account of turning around and going home when that's was correct decision.
On the final mountain, after failing to summit twice before, with a film crew, heavily sponsored, and leading a team of experienced climbers, a load of press coverage, after waiting for weeks to get the right weather they get about 200m (600ft) from the summit and ... go home. The man in question decided it was too dangerous to do any more they knew they were miss the weather window for a repeat attempt. No fanfare, no story of success, go home, tell your funders you failed to summit, survive.
I'll try and look up the book later I'm sure I've misremembered some important details and can't remember his name.
It started with practice handstands for 10mins without any real plan other than that, It then built into a similar set of small things. Short duration but required focus. Brushing teeth with my other hand, small bits of CoZE exercises - silly things really. But it have both of us the real feeling that we could get better at anything.
It was when I was travelling and I kept up a version for a month or so. Stopped because I was working on more valuable goals when I got home.
This is a wonderful cooking lesson and a fantastic introduction to spaced repetition. Might be a fun next step.
Even just pairing up and running through these items for the 1-2 most important goals of the month might be quite a big boost. I would be up for trying that / organising something around that (UK time-zone).
I wanted to pick on on the point about heritability.
I've been struggling to explain to others that despite self-control having a high genetic component (you say a heritability of about 60%) it's still possible and valuable to improve it significantly. You analogy with strength training was a really useful framing. The heritability of BMI/strength is about the same  as for self-control.
I guess the difference is that people don't join clubs specifically to train their self-control for 1h three times a week, with experts to guide them along the way and make sure the difficulty level and progression is right for them. It would be great if this existed though. I've played with the idea with a friend 11 years ago, we would have a list of mostly pointless skills that we would train, for the sake of training our self control, every day. It probably wasn't the best format, and I have no idea if it was causal but what followed was the most productive period of my life. We also ran a much watered down version at some of the LessWrong London meetups a long while ago with less (but still some) success.
It makes me want to try something similar again. Because I think the benefits of improving self-control are huge, Even if 'all' you end up with is a solid model of your own motivation for a task and how it will changes with situational changes (e.g. relative motivation of studying at home vs. the library), then it would still be hugely valuable.
 https://onlinelibrary.wiley.com/doi/abs/10.1002/gepi.20308 "Additive genetic factors explained 81% of variation in height, 59% in body mass index and 50–60% in the strength measures ... a study of one million Swedish men"
Good idea - here you go. https://docs.google.com/spreadsheets/d/1CrdJ3KwWWsDomgRqwSOn96Ix58VHUz88YQhAfiGQuBI/edit?usp=sharing.
I've found micro tracking of my time over a week long period really beneficial. I don't think the same would be true for diet as there is so many variables so look at, picking any one thing to look at would be privileging that hypothesis to an unreasonable degree. That said if there is something specific you are looking for, where you would expect short term fluctuations to be important, then I could see the value.
I suppose I would also see the value if you were running a trial on yourself. Toss a coin at the start of the week if heads each X every day, if tails then don't. Measure something you expect to change over such a short period of time (e.g. sleep, subjective energy levels)
I find Cochrane reviews to generally be of good quality, even if that means there findings are very often "we have reviewed all available data as of X year and we not able to draw any clear findings". Depending on your technical knowledge it might be useful to point out that they generally include a "plain language summary".
It is important to note that not everyone agrees with their findings (the abstract of https://pubmed.ncbi.nlm.nih.gov/16052203/ is well worth reading, not just as a criticism of Cochrane but as a comment of the field of research in general). I suppose one could reasonably argue that a combining a load of crap observational or small RTCs (with high drop out / low protocol adherence) is not going to teach you very much, yet this is what systematic reviews of the field tend to do.
You do occasionally see nice articles like https://pubmed.ncbi.nlm.nih.gov/32144378/ which follow 142 people for 2 years. It's still not a lot of people and not that long to draw conclusions for many different lifestyles and cultures across the entire human lifespan and across the world.
See https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD002128.pub4/full for an example of a Cochrane review, and for a list of advice see https://nutrition.cochrane.org/evidence
Or see this almost comical example, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7427685/ "12,133 records identified, 30 studies met inclusion criteria ... despite the large number of trials included in the review ... evidence for primary prevention on clinical endpoints is limited to one large trial with methodological issues ... [hence] there is still uncertainty regarding the effects of a Mediterranean-style diet on clinical endpoints and cardiovascular disease (CVD)"
I also second peoples point about examine.com - it's good.
Also, it's not really directly answering the question but I want to rant. Calories-in must equal calories-out but in a way that has so many caveats that it's not really a useful measure of anything.