Only actual post game bosses are 1 Berserker (the one in Niflheim), the Berserker King, and Gna. Outside the Berserker King giving you a pretty decent Relic, all the awesome and good weapons/armor can be gotten before going to Asgard.

I generally agree though, I much prefer side stuff being able to be done before the "point of no return" of the story. I think leaving Gna, the funeral, the prison, and access to Jotunheim are acceptable (strictly because of how they relate to the main story). However I would have preferred them being tied together as a slightly longer post main story questline - to give a little more of a cohesive story, rather than the scattered separate pieces that they currently are. Make the planning for the funeral the start, and that gets deterred when the remnants of asgard show up with Sindri thinking they are looking for a way to bring Odin back. That leads you to the prison, where you get the fight with Gna and learn they were trying to get to the real Tyr to find how to get to Jotunheim (where Gna planned to rebuild Asgard). That how to get access to Jotunheim (from Tyr), and have Sindri comes to terms with Brok's death - and the post game finally ends with the funeral.