It's as simple as that (no proxy, no lib...):

(((obj.a ??= {}).b ??= {}).c ??= {}).d = value;

I am eventually providing the answer to my own question because after 4 months, neither @VLAZ nor @jcalz have added an answer that I could officially accept. Thanks to both of them for providing me the answer in the question comments 🙏 All credit to them!