I have done some days at the parks without FPs in advance and had a great time.
I specifically want to address your point about not getting any "worthwhile" FPs. What attractions at MK do you consider "worthwhile"? If you are looking for 7DMT or meeting Anna and Elsa, you probably won't get those unless you're lucky. Same for the parades and Wishes, but those are easy enough to see without a FP. If you are interested in things like the mountains, Haunted Mansion, Jungle Cruise, and several others, it is likely that there will be a good selection, especially if you plan to stay in the park all day.
My suggestion to you would be:
1. Buy your park tickets now and link them to a
MDE account.
2. Make your 3 FP reservations as soon as possible. Don't agonize over them, but pick out 3 attractions you know you would like to do. If you want to change them later, you haven't lost anything. I would make the FPs for no earlier than 2-3 hours after the park opens.
3. On the day you visit, try to arrive about 45 minutes before official park opening. Getting there any earlier than that isn't going to help much, but arriving 15 minutes or more later will slow you down.
4. When the park opens, go do some of your favorite attractions while the lines are still short. After a couple of hours you will be ready for lunch, using those FPs, and doing some of the "anytime" attractions that don't draw long lines.
5. After you have used your 3 FPs you can go to a kiosk and see what else is available. You might also want to see the FOF parade at 3 PM.
You get the idea. If you plan to stay until the park closes, you might also be able to do some of your favorite attractions with short lines in those final hours.