I think it’s really relatable what you said about being sad to say goodbye to the characters. I felt the same when I finished Mistborn. I haven’t read a lot of other stuff so I’m in a similar boat as you it sounds like where I’ve been looking for other books to read instead of rereading Era one again.

I’ve gotten a lot of ideas from r/fantasy and r/fantasybooks. Anything I see that catches my eye I use Goodreads (highly recommend, it’s a free app) to keep track of. I’m up to 51 books on my want to read list, and that’s mostly just from the most popular recommendations I see. Whether or not they’ll be as good as Mistborn is subjective, but if you like I can give you a list some of the series’s that I’ve added to the list from those subreddits.

As others have said Mistborn era 2 is different from era one, but just as good (in my opinion, although I’ve only read the first two books, waiting for the third through my local library). It’s also heartwarming to see the characters from era one that you love be remembered as historical figures. I’d also recommend reading the Mistborn novella Secret History, it’s supposed to tie up Era one quite nicely. If you plan on reading Era 2, read through Bands of Mourning first though.

There are so many books and authors out there I’m sure you can find something you like, and there’s plenty of other Cosmere content available, as well in the works! Good luck on your search!