On this particular rocket, I'd recommend a different approach.
I think what you have done is structurally sound. I'd get a tube of bondo and goto town, then sand it smooth slowly.
For future fillets, spoons are great, PVC pipes, etc. there are a few tips I've picked up here I'll try to get good photos of. I'm gonna blow this thing up with photos, but it'll show better detail at this size.
First, I use a sharpie marker to "paint" the small PVC pipe I use. Then I rub it in the fin/airframe joint to make lines. You could also use carbon paper.
Then, I slowly put down painters tape, careful to make sure the edge is down. I place it on top of the line the tool made. this will make sure the excess is on the tape when you pull, and not on the airframe leaving a ridge.
I don't have photos of the next step. I use a popsicle stick to lay in the epoxy. starting in the middle of the fillet. You can use the stick to push the epoxy around and make sure it's evenly distributed. don't overfill it. Too much makes it harder. If you pull and there's a gap, you've got time to add more, and pull it again. I load starting at the top of the fin, and typically leave the last inch or two either bare, or with very little. By the time i've pulled that far, it fills in. I start the tool laying right at the front of the fin, where it meets the airframe, then pull it down the airframe in a slow steady manner. Dip the tool in a cup of alcohol, and wipe it clean with a rag. Then carefully peel the tape. Wait a day then rotate.
When making batches of rocketpoxy, doing it on a flat square of cardboard and letting it run out into a flat pool can help minimize bubbles. I do prefer proline 4500 for fillets, but I've used rocketpoxy with some success and only a bubble here and there.
some other photos-
Leveling the fins-
Sanding down bondo is super easy with helpers-
ah ha! found one of the sharpie/ line drawing process
more tape-
screwed this one up-
let it cure, then fixed with epoxy