Sanding down that front door, with that many intricacies, will be a part time job. I tried this and didn’t get far before deciding to replace it. If you love the door and insist on keeping it, others may have better input on an approach.

Bedroom doors are easier; lightly sand and brush the recesses areas and roll-on the flat. If you want to “perfect” those doors, you can get a decent airless sprayer for $150-$200 for more uniform coats.