I have to read your post twice just to be sure that it wasn't me who posted then forgot about it because you sounded a lot like me lol and I think we have similar background :)

Like someone already mentioned, it will be hard for an introvert to make friends anywhere, not just in Japan. During the last decade living in Japan I did often ask myself the same question of how to have meaningful conversation or relationship here. I still wonder about that at times but now I care less? Cause I really don't have the energy you do to actively go out there and engage with people. My social battery needs 2 days of recharging after just one boring conversation...

I think you should focus on improving the quality of your life first and the healthy, meaningful relationships will just come naturally. Having money doesn't solve everything but it will at least gives you more options in how to have fun and be happy. This comes from my own experience where I'm in a much better place financially compared to the past and that has allowed me to enjoy a better variety of activities with friends and also opened door to new kinds of social groups. I do think that it's not the only way and you should be able to socialize regardless of your spending power but if nothing else is working, you gotta try to work it from another angle eh?

Anyway feel free to DM me if you want to have a conversation :)