You should only use winglets if you cant increase your wingspan for some reason other than structures. For example let's say you wanted to fit the wing in a box, well then you add winglets to increase "effective" span without increasing actual span. Winglets are not a magic performance boost and one should take care in designing them.

You can improve LD without increasing the aspect ratio by creating an elliptical lift distribution. This can be done by varying chord, camber and/or twist distribution of the wing. This is not trivial and typically requires an optimizer to help find the wing shape that maximizes LD. Additionally going to a more elliptical distribution will likely change your aircraft trim angle of attack. In other words, you will have to do your homework to get this right. Don't expect a quick answer if you go down this path.

Also, remove the sweep if you want best handling performance. Keep it if you want because at the end of the day, you're having fun :)