Hi,

It’s pretty simple that you have a clutch that’s worn and dirty up in between the flywheel and the pressure plate.
First off, That is a lot of miles and years on a clutch system. There is no getting around it.

I have two cars in similar condition with lots of miles and years on them.
One car the clutch doesn’t slip under load but is more susceptible to your symptoms. Noticed more after it sits but driving it a lot more daily helps it work better until it can become a non issue. That transmission has over 350k on it. I’m the original owner and it’s on its second clutch.
My second car that slips is just worn out. I’m the second owner. It might be on its second clutch with only 189k. The Difference is the in drivers I would say. One is a 1978 and the other is a 1984. You are in the middle, sort of.(:)
Both have started to develop the problem of stiff or getting into reverse.
On both cars turning off the engine stops the input shaft and allows the reverse gear.
Whatever you do be gentle! Turning off is the best option, temporarily.

Don’t jam the shifter into your forward gears.
With this clutch condition going on it will damage the teeth on synchronizing wheels.
These are made of a special bronze type metal that’s not made to be put under a high stress like the gear wheels.
They are thin and light weight to spin up freely with collars to quickly achieve aligning speeds to gear dog.
Keeping the fluid freshly changed is the lifeblood for the synchronizers.
Having a fresh clutch system stops any “clipping” or clicking feeling from coming up the shifting stick on the shifting into the forward gears. If you feel and listen to the transmission will tell you it needs a fluid change. No more than 30,000 on the M46 with or without an overdrive. I prefer less because of the overdrive.
Being dead stopped and having issues is not an fluid problem at this mileage and years.

The fiber disc is a combination of composite materials that can contain various formulations of powdered metals to achieve wear resistance to heat just like brake pads.
Dusting is invariably going to an issue over time. Our brakes get rinsed and blow dried regularly.
The clutch is inside an enclosure to keep it out of a weathering environment but it has its drawbacks for any kind of maintenance.
I have thought of a pressure washer treatment.
It might help but I don’t know if it’s a common maintenance practice?
Trust me I have been tempted to try it, but mechanical wear, is not a “wash and wear’ garment thing! (:-)

When a car sits a lot the accumulation of dust absorbs humidity. That stuff gets where it wants to go.
Then, It can cause some rusting action to what should otherwise be smooth machined surfaces.
This also affects the input shafts splines and the internal splines of the fiber disc, that is supposedly meant to float in between the flywheel and the pressure plate surfaces.
The splines get choked up by sitting outside in an open environment quicker too.

Moisture does get to have its way with an old clutch.

So there you have my thoughts and experiences.
Hope it helps guide you to a solution.

Phil