Is it normal that our Wordpress' database crashed multiple times in this month alone?
Don't ever use MyISAM. Ever. Under any circumstances. Just don't. If you need its performance characteristics for some reason, use MariaDB with Aria tables.

MyISAM is not crash safe, and will corrupt data when it breaks.

Also, my webserver with WordPress has over 5k posts and has never crashed its db.

This is not normal behavior.