I had a friend that had the same problem, this year he built a cage around his seedlings using hardware cloth,
https://www.lowes.com/pd/Blue-Hawk-Actual-10-ft-x-3-ft-Steel-Welded-Wire-Rolled-Hardware-Cloth-Fencing/1000367331 , problem solved. Just make sure it's fully enclosed mice can climb up just about anything.
You'll never get them all with traps, obviously the mice got into your attic somehow and they will just keep coming.
The link I posted is just an example, you can get hardware cloth in different lengths and heights, and in different size mesh openings, any hardware store should carry it. I would go for the 1/4 inch mesh, a mouse can crawl through 1/2 inch.
A mouse won't chew through hardware cloth, I used to make cages for the rats out of it and rats never chewed through hardware cloth.
Or if you are not into building things, and depending on how many seedlings you have, you may be able to buy a mouse cage to put your seedlings inside of. Then mount your light over the cage.