One final thought for now at least. 

Light issues can often be resolved by covering only the top part of the window. We have venetian blinds in some rooms because of this.

Whilst curtains usually operate from left to right they can work top to bottom. So you could have curtains that you lower to cover the top portion of the window to address the offending light but leaves the lower part of the window exposed for the reflections. This might give you the best of both worlds.

Whilst blinds could be an option they do have their own reflective issues and don’t sound like a good fit for the environment that you have described anyway. 

Would a wood or plywood panel be better for reflections?

...and if I replace the door with a wooden one, would it be better for audio reflections?

Duplicate content, the board was playing up.

Final though for now.

Curtains can go from top to bottom as well as across and light problems are usually from the top part of the window for most of the day. So you could have drop down curtains and only cover the top part of the window most of the time leaving the lower part to reflect sound.

Venetian blinds could work but I don’t think they are suitable for the environment that you have described and they do have their own reflection challenges. 

It would be very similar i.e. most people would not hear the difference from just the surface.

As ​@Stanley_4 said resonant rattles are usually the main issue with doors and windows. I have installed silicone stoppers on a door that was rattling with the Sub.

Thank you all so much for now :-)

I'll see about swapping a wooden door I have in one room for the glass one in the home theater room.
 

Sadly "replace the door" isn't a simple option either. Wrong door, like a plastic coated, Masonite sheathed, hollow core one (like I have here as inside doors) or a hollow core wood sheathed one is likely to be far worse than any glass you are likely to have. A solid wood door, if properly mounted and the correct closing bumpers also installed properly would be about the same accoustically as the wall.

168 cm from the end of the Arc is quite a ways, you need to factor in the glass/wall areas and respective distances too. A meter door in a 5 meter wall is going to have a small impact, a 2 meter door in a 3 meter wall, much greater impact. Where it is positioned on the wall matters too.

I am thinking someone has been having fun at your expense by providing all the misleading reflection information that has you so concerned. Given even the minimal room information you have provided I'll bet you could likely replace or cover the door with anything and hear no difference. 

Still a nice room darkening curtain sounds like the right accoustic solution to me, how thick you should base on your testing. If you have the skill to mount them, narrow (2.5 cm) aluminum mini blinds mounted to the glass (not the door) and set as close to it as  possible would work, minor accoustic impact, some room darkening but with light leaks would be the downside, upside might look cleaner than a curtain. 

Given all here I'd really say do nothing but block the light, a simple sheet of butcher paper will do, until you have identified an audio issue that needs corrected. If the paper is adequate then a thin light blocking curtain would serve well.

Thanks so much! I'll do some testing, considering your suggestions!