Lettuce is highly inbreeding. In my garden, even though my fields are filled with wild lettuce that is flowering at the same time as my domestic lettuce, I rarely find any crossed offspring. This year, I only found one obvious hybrid among many thousands of domestic and wild lettuce weeds. I found about 2 last year.
I expect that pattern to hold in most gardens. So you should be able to save seed for years with the same great results as you got this year... Just be sure to taste each plant before saving seeds from it. I tie ribbons to plants that I want to save seed from, or I put a stake by them. Then I try to weed out the non-desirable plants before they flower. Hybrids with wild lettuce look much different than domestic lettuces, so if any do appear, they should be easy to chop out early on.
Last year's possible hybrids.