Thanks for all the suggestions. Have settled on a method that's partly from me but picks up on a few ideas above:
- when I start something with more than two steps, it gets an entry in my projects list (and in the notes for that project, I'll set out all steps I can foresee for now)
- most important bit: any action requiring subsequent action that currently can't be scheduled or listed - and whether it's in my calendar or next actions - gets flagged with a certain character to remind me that it's not done yet, and to therefore schedule/list the next action (probably by reviewing the project notes)

That helps me overcome my biggest worry: that I finish something, tick it off the list with relief, and then forget that there's more to come.

Works well in Outlook thanks to automatic formatting. (Which I've found pretty useful in itself.)

Thanks again!