I love 3d printing for my layout! I've had a Prusa MK3S upgrade for about 1.5 years now and its a great machine FWIW.
I definitely use PETG for functional parts, but I stopped using PETG for models and strictly use PLA. CA glue just doesn't work well on PETG and that's my preferred method to assemble. I use Zapagap or whatever else I have laying around. I have also had better success with finishing PLA. For your printer, you will have much more success with PLA.
I always prime my PLA prints before paint. Someone else said it before, line-up pins are a great method to keep things clean for assembly. In regards to thickness, ,3 is plenty thick. If you want to improve your strength just add a perimeter layer and make sure your infill is reasonable. I use 3 perimeters with a 15-20% infill. Never an issue on stuff I printed over a year ago.
Here is a model I just finished printing before I go to paint and finish. It's a Bungalow from Thingiverse that was designed for HO, but I just scaled up. The roof is 2 pieces and I will just make a jig and glue it together with CA glue.
Working on a functional RR crossing with sound for my son right now.
