First thing is to take a picture of the orientation of the discs for later reference,
Then unscrew the agitator, one side is left hand thread and the other is right hand thread, then remove the disc, then remove all the 6mm bolts and pry apart the two housings you will then have the shaft...
Have one of these collars on my TM 175.
Was on there when I bought the tractor years ago, always wondered was it a new holland part. Would love another one as mine is starting to break up internally now.
Had this exact same problem with a new holland combine, went through all sorts, ended up being the starter relay, so maybe try powering the starter solonoid direct to rule this out.
I find these guys prices are hard to beat.
Bought a fair few pieces from them now and never had any problems.
https://www.sgs-engineering.com/m18fmtiwf12-502-impact-wrench-charger-batteries
Most machinery operators should find themselves pretty ambidextrous when it comes to all this, left foot brake, right hand gear changes , left hand shuttle!!
Pretty adaptable lot we are...