Planning extra long stay for study and travel

I am a Dutch pensioner planning a long stay in Japan in 2023 that includes travelling across the country in a campervan as well as staying in Tokyo for at least one month and enrolling in calligraphy and language courses. (At this moment I am studying Japanese and aim to do JLPT5 in December.) The trouble is that I fear 90 days is not enough for everything I want to do, and I wonder in what way I can qualify for an extension of my stay for a longer period of study. (I am a Dutch citizen and no, I do not have 30M \!) Getting a workable itinerary together within 90 days is a real puzzle that would become a lot easier if I could extend the stay.

Having a car in Japan while you travel significantly opens up your travel opportunities if you are mainly outside of main cities. However, a campervan will significantly impact you ability to travel as almost no location will have parking outside of caravan parks

Take a short flight to South Korea and return the day after for a new 90 day period?

As PP said the only option is to leave Japan temporarily (eg have a few days in Korea or TAiwan) and return. Unless you do this repeatedly it is not a problem.
Unfortunately with Dutch nationality you are not eligible for the 90 day extension as a few other European countries are.

Regarding the camper van, they are much , much less common in Japan than in NL. As PP said you are limited on where you can park. Also consider that most streets are quite narrow, so you might not be able to drive it everywhere. Plus traveling by train is actually very nice.

What I would do is to consider a car (or even a camper) only for some areas of Japan. Definitely better for a city, but for regions such as Tohoku, or the interior in general a car can be good.
But what I wouldn�ft do is renting it for all the trip. Specially long distance is much nicer by train.

Sorry, the word TRAIN was missing in this sentence
Definitely better TRAIN IS for a city, but for regions such as Tohoku, or the interior in general a car can be good.

I would agree with most of the above responses.

You just have to visit 90 days at a time. Fly or ferry to Korea and return the next day, to begin a new 90 day period.

Also camper van is really unsuitable for a full nation wide tour. I can see a van being suitable for Hokkaido, northern Tohoku, Chubu, northern Chikogu, Shikoku and central Kyushu. But city access and your ability to find a parking space is very limited in urban areas like cities.

Worth investing time into a strong plan to see "if" a van is going to be suitable with all the negatives that come along with it. Or whether you can do a mix of trains and vans. Eaxample, you could use public transport and hotels for Tokyo/Kanto area, then move up to Sendai/Morioka and hire a van, then tour northern Tohoku and Hokkaido. return van to same location, and use public transport to more onto your next region.