You can never research too much. Well, not really.
You can get lost in YouTube Hell: going in for videos on sword work and realizing you’ve migrated to watching kittens dancing to Timberlake.
You can start reading articles on sword work in Wikipedia and realize you’re now reading an article on nuclear fission (true story).
But in terms of backstory (stuff you will probably never use), you can never research too much. Know your subject matter. Know the character. Know the setting. You don’t need a degree in something in order to write a believable character. You just need to know enough to write about it. Your comfort level included. For example, I will never be comfortable enough to write a character who is into math. I know this. I can research all I want and it will never take.