I understand the atmosphere is used to slow the vehicle - it's basically free brakes that you don't have to carry with you. I never suggested using rockets in reverse to slow the vehicle down. What I am asking is, instead of effectively standing on the breaks and generating enormous amounts of friction in a short period of time, why can't the vehicle ease onto the breaks and spread the friction out over time so it can be more safely dissipated (via a more shallow reentry angle).

The shallower the angle the less energy you lose, but you are still losing altitude.

At some point you lose enough energy that your speed drops enough that your altitude starts dropping significantly. You can't lose the energy without losing altitude, and once you lose altitude you start losing energy whether you like it or not

I think what you are wondering is "can I stay in the thin atmosphere bleeding X Joules of energy for 50 minutes until most of the energy has gone rather than entering more steeply and bleeding 10X Joules for 5 minutes"

However once you lose energy, you lose your altitude, and as you lose altitude the atmosphere thickens and you start very quickly losing 5X, 10X, 20X joules every minute.

See lift to drag ratio. To get enough lift to maintain altitude you need a certain amount of drag. At those speeds the drag causes the heating while still not producing enough lift to stay up.

If you had extremely big light weight wings it would help, but the materials that can do that don't do well when heated up

You also run into issues of what do with the wings on the way up. You can't just put huge ass wings on that thing. You likely need it deploy-able.

And then the wings would also survive the flip and vertical landing. Or if you want to land like a plane, then you also need landing gear.

So there is really no way to add wings without adding a huge amount of mass. You are building a completely new thing.

There are some super cool mega-space planes designed in the 70s (I think). But of course these were never built or even tested. I remember they had some overlapping metal heat shields and a big ass delta wing. They would also start vertically and use air breathing engines.

They already use a shallow angle. There's just a lot of energy involved. As soon as the drag kicks in, the angle gets steeper and steeper on its own as the drag slows the craft down.

I guess this sorta makes sense - the slightest slowdown starts to deorbit the vehicle, at which point a particular descent rate becomes difficult to maintain?

Its more than double.

It's mind-boggingly more than double. The rocket equation takes no prisoners.