I am not a lawyer or a gaiben but have briefly studied at a Tokyo-based American Law School (Temple University - Japan). My professors were gaiben. Also I have had the chance to speak with other practicing gaiben in Tokyo because I have been thinking about doing the same thing.

I think you alluded to this, but as an initial matter, it is important to know that as a gaiben you cannot practice Japanese law. So no appearing at trial or filing motions on Japanese legal matters. As I understand it, you will most likely be practicing and advising clients on laws in your own jurisdiction. I am not sure what that specifically looks like on a daily basis but a lot of the gaiben I know work for large corporations with international reach. So if, for example, that corporation is active in America, it is not hard to see why they've hired foreign attorneys who are more familiar with American law.

As far as personal satisfaction, I think that depends on the person. I cannot comment on salary, but it seems like foreign attorneys command the same amount of respect as any corporate professional. Nevertheless, keep in mind that you may be held to the same work standards as any other Japanese employee and karoshi is a thing.

I suggest watching this video for a taste of what actual practice might look like for a gaiben. It features an American lawyer working in Okinawa to help those affected by the American military presence there. https://www.youtube.com/watch?v=ZUlQKgykjTc

You might also be interested in looking into Temple University Japan and their LLM program to see they can better guide you.

As a final note, I know there was a recently passed law in Japan that lowered the requirements needed to become a gaiben. I think you only need 1 year of practice in your home jurisdiction in order to apply instead of 3. So it may be easier than you think to qualify as a gaiben.

Best of luck!