Same here, English 2nd German 3rd. I’ll never get nearly as good at German for many reasons. Mostly I’m way more exposed to English, and I use it every day.

mouse mice, ox oxen…

No I don’t have a source, it’s what my German teacher claimed. So maybe not an unbiased source?

Funny though German has the more complex rules on the surface, English becomes the more complicated when counting all the exceptions.