There are two things you can do with a lettuce seed crop that is covered in mature seeds decorated with little fluffies:
#1. Come in with a bin, or bucket, or paper bag. Lean the plant into the container and shake the the plant to collect all the ripe seed. Next week, same time, come back and repeat. Do this again the following week if needed. You can harvest every last seed as it reaches peak maturity.
#2. When about 75% of the seed is ripe, you can cut the plant just above the soil and lay it on a tarp somewhere out of the rain to deal with later. Some seed will shatter to the ground before you can harvest the plants. And though some seed will keep maturing once the plant has been cut, they won’t all ripen.
With option #2, you won’t get every last seed to the finish line but it takes a lot less time than many visits to many plants.
When you’ve got a field full of drying seed, and a barn full of garlic to clean, and maybe some CSA veggie baskets to pick and deliver; you should use that time wisely and let a little seed fall to the ground.