Six trains per hour would be fine on TPE - no problem - the problem is when you try to make those six trains a mixture of fast, medium and slow (so they catch up with each other, create conflicts).
The NSE area that you refer to can squeeze many more services per hour out of paths (than Regional Railways in northern England) because everything is a 100mph EMU so trains can run at similar speeds.
I wonder if the answer to TPE is to make three of the services semi-fast east of Huddersfield and the other three semi-fast west of Huddersfield, so that every service would take around the same length of time from Leeds to Manchester...
- xx:00 - Leeds - Huddersfield - Slaithwaite -Marsden - Stalybridge - Manchester Victoria
- xx:10 - Leeds - Cottingley - Morley - Dewsbury - Ravensthorpe - Huddersfield - Manchester Victoria
- xx:20 Leeds - Huddersfield - Marsden - Greenfield - Stalybridge - Manchester Victoria
- xx:30 Leeds - Morley - Batley - Dewsbury - Mirfield - Huddersfield - Manchester Victoria
- xx:40 Leeds - Huddersfield - Slaithwaite - Greenfield - Stalybridge - Manchester Victoria
- xx:50 Leeds - Morley - Batley - Dewsbury - Deighton - Huddersfield - Manchester Victoria
...four or five intermediate stops on every service balances journey times, so that departures from Huddersfield would be roughly five/fifteen/five/fifteen minutes, but the slower services would have sufficient time leaving Leeds without getting in the way of the faster services over that bit of track.
Trying to mix expresses and stoppers seems a recipe for disaster though. If we want to squeeze proper fast services over the TPE line (that only stop at Huddersfield) then there's not going to be much space for slow Ale Trail trains stopping at every station they pass through.