Running

"threshold" for running means maintaining an HR of around 175.

I'd like to get to the point where I can sustain that effort level whilst cycling- currently my legs get fatigued.

Just a case of working on raising FTP? Looks likely to me, just wondered if I had missed something.

More power = more oxygen use = higher heart rate.

This is going to be true of both running and cycling. But if you are spreading that oxygen use over more muscle tissue. Youre not going to hit lactic threshold so quickly. Thus you can keep increasing your HR a little more, and your effective HR max will be higher.