I think you are right that Thrips are involved amongst other things. It seems that when one insect or disease picks on a daylily...more do. Besides leaf streak...earwigs make leaves look like that, too. I find here in the warmer climates, usually you would have more than one thing going on. Root knot nematodes are more prevalent in the south...the longer a plant stays in place...the more likely they are to occur.
When not in bloom or the heat of summer, it would not hurt to dig the plant up. Wash it off. Check and trim the roots. Get your hands in there and check above and below the crown. Cut the fans as if shipping. I peel quite a few leaves off...looking at what insects are living in there. Then soak in some water with a little hydrogen peroxide. Recheck the plant for insects...let dry...replant with surrounding soil refreshed with compost or manure. Maybe all that will yield some extra clues. Then see what it does next year.
Besides the regular leaf miner I have been disturbed to find that there may be another leaf mining pest...you can see the thin leaf miner which has been the normal pest for years. Now I am seeing "Super " leafmining on the daylilies...
Some leaves have both patterns but most have either the old type patterns or the new type which is usually more towards the center of the leaf, much straighter, and almost 6 times wider. I have seen some odd silver shades in the center of leaves...that has me paranoid that Gall Midge has made its way over here. The leaves with Super leaf mining marks have been newer leaves...I would have to really rip a daylily apart to try to find something.
So besides thrips in the second picture...between the out of focus flower and dried blooms is there any possibilty of Gall Midge, also?
Roy ...I hope your daylilies recover and....
May everyone have a great gardening season!