Just take 2 piece Boolean Union (more prudent)
Then Last piece with this new piece Boolean Union :)

But how this crazzy unity and measure ?
there is a lot of décimal :)



But I am perplex about a joint "Mitre"? have you an example ?
It's a trim ? Tenon + Mortise ?

As pieces are symmetric
You have interest to take small pieces then make an extrude when you have finished ;)
So very more easy to manipulate!