What are the easiest ways to stay in Japan for at least a year minimum?

Hello!

Canadian born male here. Currently 27 years old. Web Developer for 14 years. Working remotely from Canada, US, Australia… mainly just places I go to visit for fun.

I really do like the culture of Japan, the hospitality, the food, architecture, people and much more. After visiting so many times I’ve made a lot of friends there that I still socialize with even when I’m in the home country.

I’ve been learning Japanese for 10 years now and my level is roughly N2.

I’ve been to Japan 6 times in the past few years and I’ve been dating a Japanese citizen for about 8 months now. She visits me, and I visit her. She just left a few weeks ago.

I can work anywhere and I currently make about 100K /year.

I’m heading back out to Japan for 90 Days from this November and my girlfriend recently asked if it would be possible for me to stay there longer.

I have no problem staying there longer but I do know the tourist visa only runs for 3 months max so I decided to ask you guys:

What would be the best method to stay in Japan for at least a year in my situation. Even if not during this next trip but possibly for next year.

Thanks.