My kid’s school had them start English in grade 1, learning both languages in parallel, with English being complete immersion (not a word of German spoken and even in a couple of cases hiring teachers who spoke no German). The kids learned both languages at the same rate.

My story is just anecdote, not science, of course, as is yours.

I noticed that a number of the German parents had the same concerns as you, because they expected the spelling to be more phonetic. German orthography, like Spanish’s has changed over time to match pronunciation shifts. English does not. As a result English words often hold their roots. The trade off is that you may not knownthe pronunciation of an unfamiliar word but may be able to figure out its meaning. As with all languages, in the end none is “harder” than the other despite folklore: sharp corners are always being knocked off because the tool is so important and in continuous use.

In the most recent German spelling reform, the authors were concerned about this trade off.

Another way to describe it: you can’t approach writing c++ as if it were Java, and vice versa. Quite similar, yet quite different.