One of my old coworkers lived in a whole neighborhood where the builders ran the main supply lines for every house under the living room slab, some sort of questionable plastic line (I don't remember what kind), and the whole neighborhood basically had all their main supply lines burst under their living rooms over about a two-year period.

My point is never underestimate the corners builders will cut to save like $50 or whatever even when they're building an entire house.

Honestly taking out half of it is probably not too different from taking out all of it. A single layer of thin steel like a garage door provides basically zero insulation. In a lot of ways you might as well have a window open.

Doing it yourself is seriously super cheap if you're willing to DIY it a bit instead of using pre-cut-for-a-garage-door kits. I have a small one-car garage and by cutting a foam insulation panel myself and installing it with foil tape I did mine for like $30-40.

The trick was doing each section with two pieces, side by side. Each one scored and snapped horizontally so I could insert it slightly folded and when I pressed it in it locked into place against the top and bottom of the panel. On my door there's a small void one one side of each panel, so I slid one piece into that, then inserted another next to it.

It looks great. It works great. Makes the whole garage quieter, even. Dampens rattles when the door is moving. Weighs maybe a pound or two.

Edit: If you're looking to keep a garage warm, it's also well worth checking and possibly replacing seals. It's not very hard to replace the bottom seal, maybe even install a strip on the ground below it. It's also generally not hard to replace the exterior seals on the top and sides. I've done all of the above and having my garage well-sealed has worked wonders on its temperature and keeping out traffic/city noise.

I bet a 3d printed guide will get you really close. Certainly close enough that you'll be able to pretty easily hand sand it down to perfectly smooth. Just construct the joint before you do the final sand and finish on the staff.

You might also want to permanently fix the threaded rod into one side with epoxy or permanent threadlocker. That will probably help it always land in the same alignment when you screw it together, to avoid feeling a lip.

Ever so slightly chamfering the edges where it meets is probably also a good idea. It'll make a slightly recessed ring where the sides meet, but that's better than a ledge, and will help avoid splinters.

I don't think that manual is for the same system. The layout on the receiver is different, OP didn't mention DVD-playing, and the ports on the speakers in OP's photo really don't look like those spring-loaded bare wire connectors.

It is a distinct possibility that they're some sort of goofy proprietary Bose nonsense.

Each of those 5 RCA plugs on the receiver has two wires.

You need to build or buy five cables like these, such that each of those RCA plugs on the receiver splits out into two banana plugs and goes to one speaker.

I think most shower doors thresholds like that are in fact just meant to be stuck down with silicone caulk. So yeah, just get all the old caulk off both sides and stick it back on there.

Probably pay special attention to looking for potential leaks at the ends, and make sure to get the silicone in there such that it seals it up the sides a bit too.