Troubleshooting my Bavarian Creme
I think the problem is your cream, but not your fault if your recipe didn't mention it anymore. For a Bavarian cream, you normally whip cream and put it in the mixture in the end, to make it more light and also less liquid.

In that case, I think your amount of gelatin would be enough, since you'd only have a good 250 ml of liquid (but not all gelatin sheets are as large as others etc.).

I can't say if you let it boil long enough or not. The "back of a wooden spoon test" doesn't work for me. I normally let it cook till I see it has thickened a bit.