If you dont already have one, make sure you buy some sort of house in the meantime(any small or medium will do). That way if you find your large plot open you can use the relocation system to skip the RNG lockout. In the meantime, it never hurts to check the wards with a little regularity(having some friends who know you want a bigger house helps too). People lose houses to inactivity all the time - even some of those with larges. Also some people with larges will also be willing to trade to a smaller house if its in the right place - meaning their old mansion plot is now open for the taking.