This all reads like airlock and bad thermostat.
mixtures cause problems
m2cw

When filling the cooling system a very slow pouring time can help big time.
Maintenance manuals can give you a time flow rate etc, the engineers have worked this out all for you,
As Westy points out.

Even fill 1 ltr in a say 1 minute go away to let it settle ,come back 1-2min later and repeat.
sometime air lock happens just filling system up, "slow and steady wins the race", they say.

Humm, Just a thought,
Is it possible the interior heater could be blocked and causing issues with the bleed process?.

Was talking with a self employed rad specialist yrs ago, a mates neighbour.
between us blokes over the fence,
Fortron was his recommended flushing product.
It's not caustic, a nice and friendly product, can leave in a week, even whilst driving.
I did what he said, no problem accured.
But you guessed it, cant seem to find it any more.