Visa advice

Hi everyone!
I hope there is someone out there who can give me some advice for my case.
Let me first tell my story.
I want to get married to my Japanese boyfriend who is 9 years younger than me. Our plan is to get married when he turns 18 this October and then we can officially live together and keep on studying and working.
I know it is weird to some people but to us it's natural and genuine. He hasn't lived with his family for years and tried his best to study and live in Tokyo by himself,
We have dated for almost 1 year and a half now ,so i guess there is no problem about the proof of genuine love if we are asked by the inspector. We also have our parents' permission.

So here is my problem :
I came to Japan under Student Visa, and it's gonna expire next month ( August 14th). Should I leave Japan once and come back in October under tourist visa to apply for spouse visa when he turns 18?
If I do so, is the chance of being rejected high ?
And should I tell Japanese embassy in my country that I want to come back to Japan to get married ? Or should I just apply for tourist visa ?

Or should I change my visa to Temporary tourist Visa right now in Japan so I can stay here and wait for October, and start my Spouse visa application then?
It is exactly 2 months to wait counting from my visa's expiration date. Do you think it's easy to get 2-month tourist visa if I apply here ?

I am thinking of hiring a lawyer to help but it costs a lot of money. Do you have any recommendation ?
I am really worried about the timing...

Thank you for your time !
I will feel really grateful if you can give me some advice.

- If you are from a country with which Japan has visa waiver program (meaning, you don't have to apply for Temporary Visitor - tourist - visa beforehand), then you shouldn't have any problem under normal circumstances to come back in October under Temporary Visitor visa. You have been here as student, now you are completing your studies, and you are coming back again afresh as a visitor.

Once you come back and you get 90 days, then you should be able to go through the marriage and spouse status application within that period of time.

- But from the sound of it, you sound like you have to "apply" for Temporary Visitor visa, and it is not given always for 90 days?

- In any case, please do not overstay your visa, not even for one day; please do a good research into what documents are needed for you to get married in Japan; and please do a good research into what documents are needed for the two of you to apply for spouse visa for you.