Well, actually yes.

I just put it in first, released the parking brake and I can push it back and forth a couple feet.

The free play is within spec.

Pedal assembly looks good under the dash.

It feels like a normal pedal. I press it with my hand and it’s about an inch before contact. By foot, it goes down and feels fine.

That all seems normal. Along with gap between TO bearing to clutch cover.
Good call.

I'd lean towards bad friction disk or weak cover fingers.

You stated that it slips in 1st and 2nd but not in 3rd 4th or 5th correct? Have you changed your tranny oil? Just a thought.

I changed the oil today, made no difference. But that was a good call.

And it's "better" in 3/4/5, I press the gas and it'll slip.

Here's another note...If I'm going about 25mph, I can feather the throttle and the RPMs go up...but so does the MPH. I know my actual speed is doesn't change, but the speedo will follow the RPM...-ish. And not perfectly. It'll bump to 35 and stay as long as the throttle stays put. I release the throttle and speedometer normalizes.

If I try it again right away, the speedo wiggled but didn't go up.

Next time it did it again.

Tranny fluid would affect synchros with shifting/grinding/bearings/sluggish only. But good call.

Were the clutch cover bolts lock-tighted?
They may have loosened.

I know what you are thinking of. That happened on my '84 Scirocco. The PP I removed does not indicate that type of failure. But who knows?

I thought, what if it's the wrong clutch in the box and I couldn't tell. I eyeballed both assemblies and they looked the same.

I just looked at your original post again, slipping more in 1,2 gear? If so I do not think it is the clutch slipping, load on the clutch is higher in 4 or 5th.

Correct. But it does slip in all gears.

It’s just really tough to get moving from a stop. My driveway has about 4° slope and I could barely get it into the garage. Once I’m in 3rd, it’s less but still happening.

Still sounds off to me, it should be much worse in higher gears. Can you leave it in 1st gear and lift the rear one side at a time and try to spin each tire, just checking that your axle splines are still attached properly to your diff.

I never disconnected any axles.

It’s AWD, leaving the transmission, driveline and front axles in place is much easier.

I just pulled the engine, left it on the engine hoist, replaced clutch, then reassembled.

Keep in mind, the car drove great for 35,000 miles. Then poof, no mas.

I’d have that flywheel checked out. Seems to be the only thing left to change

Have you tried adjusting the activation rod from the pedal in the image you show? Are there nylon bushings on that thing?

You did not disconnect the axle does not guarantee they are connected, apparently possible for them to slide out. You mention indicated speed increases with increased engine RPM but car not actually accelerating. Sure sounds like speed sensor is sensing transmission spinning faster but not the wheels. Check the axles.

I’ve been stuck on the thought a manual transmission either works or doesn’t. Mine is smooth and quiet so I never considered its internals being faulty.

Is there anything in a manual that can cause my slippage? Am I looking at a bad transmission?

another WAG- Slipping transfer case clutch.
It's sending power to all, then one, then all, giving the sensation of slipping.

Makes sense since he can push the car a foot or two with it in gear and not running?

I do not think it is the transfer case clutch, pretty sure that is only attached to the front drive on that car with the drive to the rear wheels being straight through. I'm still thinking rear half shaft, or somebody tried to steal the catalytic converter and attached the rope to the driveshaft by mistake and it is gone.