Hi Sara,
I haven't read much of the thread you linked as it was soooo long, so not sure if these things were covered there. But this is what helps me with my DS who is 16 months.
Apparently it's normal for babies to get smaller appetites after 1, as they are growing so much. Also the more mobile they are, the more picky they will be - I believe this is a natural selection thing: when they can't move food is brought to them by an adult who knows better, when they can move though, they can get into anything which may be poisonous, so they naturally prefer sweet things as bitter usually means poison. The other thing is that toddlers will eat for their requirements if offered healthy options, not regularly for each meal, but it evens out over a week. Keeping these 3 things in mind has helped me relax as the appetite of my adventurous eater has dwindled and he's got pickier.
I also have to remember that toddler serving sizes really are tiny - 1/2 a slice of bread or 1/2 an egg are both considered a serve! They have such tiny little tummies!
So now I try and give him the foods that will give the most nutrition/calories per serve (called nutrient dense), eg I always give kumara instead of potato as it has heaps more vitamins. Nutrient dense foods are things like fruit, non-starchy veges, meat, fish, unsweetened dairy products, eggs, good fats etc. even if he's eating small amounts of these things, I know he's getting lots of the vitamins and minerals he needs. He would happily fill up on bread, which is not nutrient dense, so I only ever offer him that if he's eaten everything else and still wants more, or as a carrier for something I'd like him to eat - leverworst for example!
The family meals sound like a great idea, I don't think 4 or 5 times a week is a problem, he's still getting the exposure and seeing you all eat and enjoy family time around a meal.
Regarding the takeaways, if you're concerned about that, I'd either make it one meal that you eat after DS goes to bed, or else ditch it. Otherwise, choose healthy takeaways (kebabs, sushi, Thai, gourmet burgers etc) and help Z learn that there are options beyond fish and chips! I'm with you on the need for a break though - I try to make double portions 2 or 3 times a week and freeze them so I always have something I can grab from the freezer when I don't feel up to cooking.
If you are worried about his dinner eating, I'd be inclined to drop the yoghurt to much later after dinner, or to use it as a snack well before dinner on the nights you eat as a family. He may not be worrying about sating his hunger if he knows there's yoghurt coming.
Also regarding the snacks before dinner, I often give my DS a snack before we eat, usually to keep him occupied while I cook and to stop meltdowns. I make sure it has protein, fruit/veges, and fats in it (usually no or few carbs as they fill him up too much). That way dinner can be more exploratory, tasting and trying new things, without him being super full from the snack or me worrying about his nutrition.
Looking at the list of things Z likes, these are the sort of snacks I would give my boy:
Apple slices spread with almond/peanut butter
banana chopped, rolled in almond butter thinned with coconut cream and then rolled in coconut
cheesy dips with vege sticks.
ham slices spread with cream cheese and spinach then rolled into cigars.
yoghurt with a variety of fruit to dip
peanut sauce with veges to dip.
Some out there ones that DS loves that you could try:
chicken liver pate on cucumber or radish slices
partially thawed peas and corn
boiled egg with kumara soldiers
smoked salmon and olives
guacamole with vege sticks
I would also try to present some of the things he likes in different ways and get variety of flavour and texture like that. Eg if he likes broccoli, try it steamed, mashed, roasted, with cheese sauce etc. That might help him transition to new foods.
I have lots of other ideas, let me know if you want more.