Summer trip for family SE Asia / Japan

Before my girls (9 and 14) are too big and cool I want to take them on a backpacking trip around SE Asia for the summer holidays (6-7 weeks, July/August). I travelled a lot in my twenties but have no illusions that this will be completely different. Back in the day I just had a lonely planet book and no plan (ah the freedom)

The big one loves to shop, the wee one loves animals, me and the missus are quite outdoorsy and we're all quite food orientedl. So, looking for a bit of beach, culture, shopping, nature.

Current rough plan is uk-japan(7-10 days), Hanoi and travel south, 2 weeks) Cambodia - siem reap (Angkor wat) (1 week?) , Thailand (2 weeks) fly home from Bangkok.

Would also consider travelling down through Thailand to KL or Singapore(£££) even.

I've holidayed and worked in Japan. Happy that it's safe, good shopping, nature etc and japanese food is our favourite but it's expensive and I'm not sure what there is for the wee one. This is probably worth a thread of it's own but how to travel Japan as a family of 4 without blowing the budget? How are the theme parks? Is there an Airbnb equivalent? Would be nice to have 2 bedrooms for a night or 2.

So, am comfortable in Japan except for budget. We can wing it after Tokyo but I guess it will be a round trip for easy flights.

Vietnam I've never visited, rough plan is to start at Hanoi and work south. Ha long / Lan ha, Hou an, nah trang, HCM/Saigon (2-3 weeks)

Is this timescale pushing it? I imagine travel will be much cheaper here so probably can book a 2 bed apartment at the beach for 5-7 days. How about safety? I've heard it's easy to get robbed and night bus/train is not a good idea.

Siem reap I know can be expensive. Is it worth booking this in advance maybe? Overland travel or just fly? I was thinking a week here, 3-5 days to explore Angkor wat (too much ?) and maybe battambang

Finally (?) fly or overland to Bangkok. I have fond memories of lonely beach on ko Chang so might swing past. Although I like the look of "I love phants lodge" and really want to give the girls an elephant experience so maybe better heading to the beach in the area after that. Then either back up to Bangkok or keep going to KL. I stayed a while in Thailand, know it's a bit corrupt but generally fairly safe (although is that true with 2 young girls) ... Then again that was 20 years ago

It's a pretty rough 'plan' at the moment 😂 but how does it sound in terms of timings?

The other main concern is safety, I don't really want to be stuck on organised tours and want some of that independence to change plans at the last minute. However I will probably book a few of the key stops in advance. My blonde female friend had a tough time in Malaysia which puts me off the overland trip to KL. Any concerns for Vietnam, Cambodia, Thailand?

Weather is also a concern, I know its going to be rainy season and remember well when my bamboo hut got flooded on my birthday (July) in Pai but we're stuck to school holidays so will have to live with it. On the other scale Japan is probably going to be uncomfortably hot at the start of July.

Finally, budget.... Am seriously thinking of dropping Japan because there's a chance we could seriously blow the budget. Any top tips for keeping costs down as a family are most welcome.

Ok ... Finally ... Destinations. Must sees , theme/water parks, easy independent travel, nature experiences ... Tips welcome and will determine the route we travel.

Imagine as we get closer to the time I'll nail this down and have a million more questions.

Thanks in advance