Answering a few questions...
Looks really heavy.
the fin can is 230 grams complete, nose is 240 grams compete. The goal here was a very
DURABLE fincan and nose. This is now well proven. Between me and the other folks I have given them to, MANY flights with motors up to an I485, recovered with a streamer or 16" chute. No damage from flight (or in my case no damage from throwing it into the car under a pile of stuff after the launch. One did come in ballistic (not mine, but a friends) the dug out the nosecone and the fin can and reused them - durable!
Fincan, could be lighter with me pocketing out part of the cylinder. Plus I could have shortened the engagement into the body tube It is currently 31 mm, 20 MM could be sufficient but by making it longer it reduces the bending moment when the fins hit the ground. All a tradeoff for durability. Relative toe the fins, I wouldn't make the fins pocketed. The thick section is only in the center with the edges flaring out to thinner sections. I would have done a full lofted shape but am a bit too lazy to do that, could have removed a little weight by doing that.
Nose - nose was designed to add nose weight, I have other versions that are around 100 grams lighter (but not as durable)
Considering you only have one rail guide I'd move as far forward as possible.
How many and where is always a debate on here. For a short rocket like this, Im good with one rail guide (different than a button), easy to add since I was printing the the fincan. For longer rockets I have done with the same fincan, I do add a rail button, ideally behind the center of mass, but i leave the rail guide on the bottom. Simplest is a to screw into a 3d printed 'C' that is slight oversized so it pushes in and holds in place, either a sheet metal screw (pilot hole drilled) or what I do more often is drill and tap a machine screw size hole to attach the forward rail button.
Not sure why you have such thick prints either. I'm running either .8 or 1.2mm with a .4mm nozzle of course I'm going for lighter weight and dual deployment which adds weight so....
Again, going for durability (heck it is in the title) A 1.2mm thick fin using a .4mm nozzle wouldn't be very durable (yeah I know it may be strong enough to fly, but durable? ) On the printer I used for making these parts, I leave a .2mm nozzle on it, I like the quality and control with it. for anything I do that is structural it is 100 percent infill, I have talked about it many times on here, if i want a hollow section I model the ribs / stringers / shear webs etc.
The other reason these are thick is stiffness, using a low modulus material like PETG, the fins want to flutter at high speeds, a 1,2mm thick fin would be SIGNIFICANTLY lower in bending stiffness and susceptibility to flutter.
I made a small flat sided feature and then tapered the fins lateral and axially from there. I purposely had the thicker section extend to the base, again for durability, tapering them down on the bottom makes them more likely to be damaged hitting the ground with a fast descent.
