Seems an obvious question but can I create a mission at home where my take off point and first waypoint is at a flying field some miles away? My first waypoint always seems to come up on the map where I power up my FC.
Yes. You can drag the Home-marker to the designation as well.
The home marker is largely irrelevant when mission planning. Just make the mission you want, wherever and whenever you want. There’s no issue at all.
Additionally, while a TAKEOFF command has lat/lng fields, they are not used. Only altitude is obeyed.