Thanks for playing!
If you just want to try any boss on any difficulty there is a boss practice mode for this.
I think giving the opportunity to start from the same spot if you die kinda kills the rogue-like element in this case, because you would just brute force all bosses with whatever strategy you came up first without trying out other stuff.
As for ability to restart, nothing stops you from quitting on the first boss (right after picking upgrades) by pressing P or Esc.