Hey there Hugo, thanks for your post and may I say you are not alone with this dilemma; I have been through it myself.
You are actually very close to the answer! When you are playing the pedal tone you are actually nailing the process and it is fortunate that you recognise the difference when playing the Low C, there is your “Point of Difference”.
Instead of being disturbed by this, embrace it. Play 1,000 of the loose pedals and learn to appreciate them, this is all part of the unlocking process.
You have been playing for a long time so releasing old habits takes a while.
I want to encourage you to persist with working through the course even though the PROCESS isn’t there yet, that doesn’t matter as long as you are spending time developing the sympathetic oscillation.
TASK: Try tiny changes in pitch from the pedal at first and I mean TINY with your EYES CLOSED. Eventually you will connect it to the Low C. IT MIGHT FEEL WEIRD!
Don’t be put off by the difference in feeling to your regular playing, it will become the new normal over time.
Be prepared to be outrageous with your experimentation to find ways to make it work, remember, you must change something to make a difference to your playing.
Again, go as far as you can through the course and merely observe when things begin to lock up and where they really start to cut out. Don’t panic about the fact that it isn’t 100% pure process, over time your pedal tone will filter through into your playing.
Stick with it Hugo, it is a wild ride but positive determination will get you there.