I would 'TRY' moving the #7 spark plug (the primary plug under the coil pack) to a different cylinder for a troubleshoot test and also move the coil. Now keep track of what was where originally and what got moved to a new location.... We have two parts in three different positions, moving. So we want to know what causes a fault to move from #7 cylinder to another location.

I am going to be hanging off the cliff by my talons awaiting your results

Good luck.

Just moving stuff and touching your truck may fix it all together or you may find something askew.
Cheers

Today I was trying to get rid of the P0688. I renewed the whole wiring, directly from the power module to each coil and injector, plus to the powertrain control module.
Error is now gone, but it still gives that bumpy moves sometimes and runs rough. A lot less now though, but still.
Now my question is, I will test it tomorrow as well, but is there a time delay, when the engine is hot, when it does switch over from open loop to closed loop? Or is it like "instant" when the engine is up to temperature? So if I would drive around a few miles, then switch the engine off and wait 1-2 minutes and re-start. How long would it be in open loop before going to closed loop again?

Even on a cold motor, it'll go into closed loops as soon as the O2's get hot. Generally 2 or 3 minutes..... Not really long enough to be able to tell for sure. On a hot motor, its probably less than a minute...

OK, yeah, the thing is, before I did the repairs today, it would start behaving like that very quick. So it might be that it is really the O2 sensor now the only issue.
Will disconnect them tomorrow and test it again.
Thanks for your help!