I've had decent luck with freezing threadfins. I just put them in a ziplock bag and fill it with saltwater and freeze them. They don't get mushy that way. Or they haven't for me. But I haven't kept them froze for over a week either I always use um up

mix up a solution of 1/4 part canning salt to 1 gallon water , soak them in this solution in refrigerator or on ice for several hrs. Drain them and then put in ziplocks ( removing as much air as possible ) or in vaccum packed bags if you have access to them .

I use zip locks and fill them up with the bag under water till all the air is out, then freeze. Use lake water not chlorinated water. I never tried it but knew a guy who put brown sugar in the bag before freezing.

I never thought about using lake water. Maybe you could just dechlorinate the tap water.

You can crush em up, freeze them and use em as chum in a mesh bag on your next outing.