Thin walls are an absolute nightmare, I know myself! It’s difficult when you want to live a normal life but also get good sleep, neither party is at fault for wanting either of those things, it’s literally the buildings fault.
I’ve been in similar situations and on both sides of this, and I most recently learnt the best way for all of us was for me to approach my neighbours in a very kind friendly & lighthearted way to discuss how annoyingly thin the walls are and how frustrating it is for both of us, have a bit of a laugh about it together & discuss if there are actually any soundproofing/structural changes we could both make for the sake of our leisure and sleep times.
in the end we didn’t event need to do anything, because after that very positive & mutually relatable convo, the negativity stress part of it went away! I still hear my neighbour bashing their chicken breasts most Sunday mornings at 9am when I want to sleep in, and hear their dog barking several times a day, and they still hear me practicing my god awful singing an hour or so, several days of the week. But it just doesn’t stress us out anymore because we both understand its not us causing the issue, it’s the house, and we are both just trying to live a normal life.
In past properties when I’ve had thin walls & both of us “complain” to eachother, in a stressed & annoyed manner it just put a massive veil of negativity over everything. It just made life miserable for everybody.
There’s such a massive difference in quality of life in when we can change our mindset & outlook on circumstances like these that are pretty much out of our control. And I know it’s not you who has complained, but just try to empathise with them if their sleep schedules are being disturbed it is difficult. Again it’s not you, it’s the house, but you can both try to learn to live with it & make things more pleasant for eachother. Also remember you guys don’t really know eachother yet, and there’s still time to turn this around & make life pleasant for both of you.
id either write to them or go round and have a super lighthearted & friendly chat, take them some nice biscuits & introduce yourself properly and make fun of how annoyingly thin the walls are and how you absolutely don’t mind when you can hear their microwave etc because that’s just part of normal life, and keep referring to how it’s the silly houses/walls fault (leave out hearing their arguments as that just adds back in more negativity to the situation).
also try to build a relationship with them, ask them how they did their garden so nice or decor etc, chose anything relatable that you can give complements on. Get to know them, ask about careers, family, and tell them about you.
Be a really friendly neighbour, kill them with kindness, and soon there will be no complaints so long as you all continue being respectful to eachother :)