I think my wifi is fine, since I can use other wifi services such as the eShop, YouTube, browser, Dream Suite, etc.

But visiting and hosting seems to be a huge issue.
I cannot host at all because the game will do it's "saving" thing before someone enters, it takes a long while, then Resetti pops up with the error message, but there's no error code for me to refer to.
It happens a few times with visiting but it will work after a few tries, but hosting will not work at all.

Is there any way to fix this? I've tried everything.

It sounds like you may have a very weak connection to your connection point. Each scenario sounds like you're getting disconnected or "lagged out". If possible, try to get closer to your connection point, and also ensure that you have no other devices that could be interfering with the connection.

I'll try that the next time I connect with someone, thanks!