Unmarried partner visa - proof of living together overseas

I'm a British citizen and met my partner overseas in her country. I was living in Australia and invited her to live with me. I wrote an invitation letter and she was granted 12 month multiple entry Australian tourist visa. In this time she travelled back to her home country twice to finish up at her university and to prepare to move to the UK with me in August 2019 as a student. While in Australia we have nothing with her name on it showing she lived at my address. The only evidence I can think of is the invitation letter I wrote. By the time her student visa expires, we will only have letters showing both our names living at the same address for 1 year and 7 months (2 years evidence is required) because when we first arrived we were just in Airbnbs while searching for somewhere to rent. I now have a full-time job and a 5 year mortgage so moving back to her home country or Australia to live together for a few more months isn't an option. Is this going to be a massive problem? If so what can we do?

An alternative to the unmarried partner (fiancée) visa may be a marriage visitor visa for which you wouldn't have to prove 2 years cohabitation. The disadvantage is that she would have to leave the UK after the wedding and apply for a spouse visa from abroad.