I would recommend looking into an aliasing service such as Addy or SimpleLogin to use a different address for every service, which then gets forwarded to your main email address. This way when you get spam emails, you know who sold your data or had a breach, and you only need to create a new alias and change your email at one place instead of everywhere.

Runbox had contacts and calendar when I used them a couple of years ago, so I would assume it still has them.

I just run some simple services such as Audiobookshelf or Wallabag behind a reverse proxy. After reading the other comments, it does not look like there would be any benefit for my use case.

Yes, I have always used a reverse proxy which seems to eliminate the need for multiple IP addresses. It seems like having multiple IP addresses just creates additional cost and complexity, but I have seen many VPS providers offer multiple IP addresses, so I was curious if there was a use case that I was not aware of.

Yes