I can't help much, but I'll share my experience which was kinda similar to yours as far as end results.
I got one-part 3M Urethane Seam Sealer #08367 from my local NAPA store (the only stuff they had). It's black like tar, somewhat thick, and I made a great mess but couldn't get it to lay down nice & smooth. Finally said screw it, it's good enough for an old work truck, and at least the gutter is sealed. After drying for several days, I painted it white to more-or-less match the roof. I wouldn't accept these results on a really nice truck, but I don't even notice it anymore.
https://www.napaonline.com/en/p/MMM08367