You mean that it's cumbersome to have it hidden behind a key combo instead of it being its own physical key, correct? I get that, because it would slow you down tremendously. At the same time, if those letters would be physical keys, it would not be a European keyboard anymore, but a Finnish keyboard. So maybe the conclusion is that this project is nice for some languages, but not for all. In my language this keyboard would work, but letters like ö, ï and é are relatively rare and I'm actually content with the solution that I already have: https://en.wikipedia.org/wiki/Compose_key. So for me it doesn't actually solve anything.

If I’m mostly writing Finnish text, then the native Finnish layout (with separate keys for Å, Ä, and Ö) seems better. And if I’m coding, I personally prefer the US English layout where [, {, ;, etc. are more easily accessible. But I could see this layout being a better fit if I’d need to frequently swap between multiple different languages and still have convenient access to the most common symbols used in coding.

This is the reason why just using local layout (or modification of it) makes the most sense. The keys are where they are for good reason. That said do you ever really need å or Å in finnish layout?

As the guy is German and most certainly uses ä/ö/ü/ß, he probably has an idea.