Relationship Visa Situation... we live in different cities!

Hi my partner and I are planning on applying for a Relationship Visa this year. But our situation isn't as straightforward as I would like.

We've been together for three years but have lived in different cities in the UK off and on due to me starting my career in London and his being in university up north. Now I have a job in a city an hour from him his university. We've discussed moving in together but that wouldn't be till next year. His lease ends before mine and I was wondering if a letter from my landlord saying he's allowed to live with me would be helpful in our application? Or if there is anyway to prove how serious we are about each other without living together yet?

I wish we could wait to apply once we've moved in together but because of my current visa situation this isn't really possible.

I really am just trying to figure out what evidence we would need to explain our dedication to each other and the seriousness of our relationship in light of us having to do long distance for over a year.

I have enough income and savings-- and we have so much proof of the nature of our relationship. I'm so scared and stressed about this situation. I really love my partner and we are really trying to figure out how to proceed.

Any input is greatly appreciated!