You mention soldering the parts together;
do you know about 3D print welding?
For reducing the layer lines, you have a few options: you can use a sandable filler primer to fill up the layers, and there's also resin fillers, like XTC-3D. Some people use wood filler/Bondo all over their project to fill the layer lines, which would definitely work, I personally just find it too messy. If you have access to a palm sander, I highly recommend one as it'll save you so much time and effort when it comes to sanding prints.
For the joint plate: have you looked at fibreglassing the inside?
It's all coming together quite well!