I must advise you to seek the services of an actual accountancy firm and anything I say should not be relied upon as advice.
Just to be clear, are you talking about coming to Japan just for fun every few months..? Or to actually come for the purposes of business? The quick answer to your question is no, your partners are not the ones that would be able to sponsor you (unless your partnership is actually registered in Japan as a GK?).
The issue you will run into is whether or not your activities in Japan will trigger Japanese consumption tax (JCT) and whether or not you will eventually create full on a permanent establishment for your company which will have corporate income tax (CIT) implications, all of this putting aside that you may have personal income tax obligations (PIT).
In regards to the visa, this is where you will need professional advice looking at all of the factors of your situation, but if you plan to go on secondment with your company as the sponsor, you would have to register your company as a Japanese entity (relatively simple to set it up as a GK) and have the entity apply for your sponsorship. The question becomes if you want to actually do this because it will for sure create a bunch of tax consequences that may not be necessary depending on the reasons for your visit. There are also other visas you may be eligible for (self-employment, highly experienced) but if you only plan to come a few times a year, say 2-3, then the tourist visa is honestly fine.
If you simply plan to come to Japan to meet a bit with your partners and spend the rest of the time relaxing in Japan, you can definitely structure your activities in a way so they don't force you to deal with all of the formalities of Japanese taxes, but note that the amount of days you spend and your business activities (this is a complicated topic) in a year are cumulative. You would want to check your country's tax treaty with Japan to check what the threshold is before someone is considered a permanent resident for tax purposes - this would have your worldwide income taxed in Japan. Yes, there are a bunch of systems in place for foreign tax credits or just outright exemptions, but each situation will be different.
This is a very broad, generic, and summarized version of some of the considerations..but in all honesty, assuming I have the right idea of what you want to do during the year, this could be done relatively painlessly. There are services in place to rent weekly or monthly mansions/apartments for business travelers and plenty of share house options. If you really want to have your own permanent rental for your private use during the year, then you are definitely going to have to consider some of the issues I brought up and seek some professional guidance.
A very basic analysis of your circumstances usually runs about $3k from any of the Big4 firm's personal advisory services.
Time-sharing sort of exists but not exactly in the same sense as I think an American would think of it i.e. owning short periods of guaranteed time at a property. The concept of time-shares here is usually like a resort or hotel agency renting a place out, basically like an airbnb. It's more a cultural thing than anything due to how vacations are taken in Japan (differences in seasonality compared to Western countries). Property and rental business in Japan are very different in general, especially once you get out of any of the major metro areas. That isn't to say you can't find a vanilla time-share, but you will most likely be looking through foreign channels rather than native ones.
It's definitely a bigger investment/giant headache to do, but buying a mansion(condo) a bit out in the inaka is quite common. A used 1DK in a more rural area can cost as little as USD $25k and can be used as your base of operations when visiting. Now that I'm back in Tokyo I rent a mansion in the area for work, but own one in Osaka where we actually live, and then we have our house out on Shikoku for family/relaxing. You might think oh wow that's crazy but to be honest I think the total values of the house and Osaka mansion would shock you (the Tokyo one would probably shock you for the normal reasons).
The issue is always where is your primary target for travel. If Tokyo always needs to be your center of Japan, then you'll look at places 1.5-2 hours outside of Tokyo, but this is totally normal and most people have a daily commute of this length. Grabbing a place 15 minutes from say Takasaki and then coming into Tokyo on the Takasaki line is a totally viable option. And if Tokyo doesn't have to be your center of Japan, it becomes even easier and cheaper.
Thanks for the suggestion though!
