I think I’ll abandon the pressure sensor approach, and maybe have a go at using this technique of hacking a Xiaomi door sensor with a float switch.
Still battery operated, but you only get a binary measurement and it is intrusive on the water tank (needs to be plumbed in).
https://www.aliexpress.com/item/32894861981.html?spm=a2g0o.productlist.0.0.a5aecf1bFQjfz6&algo_pvid=c38b6654-e0b4-4975-b3c2-e22db5a722de&algo_expid=c38b6654-e0b4-4975-b3c2-e22db5a722de-12&btsid=0b0a555516124727106762580e0616&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_