@marc-van-der-heijden
Thank you Marc, that‘s a good hint!
Now, I borrowed a different model iPhone with the same iOS version, and the problem was way less pronounced than with mine, so clearly there was a mis-config on my device and I started digging.
Somewhere, unfortunately I cannot find the source anymore, I found that third party apps are not allowed to use Siri voice and therefore pick whatever voice selected first in the language environment.
So I looked at my language environment and found several, so I deleted all but German. (This didn’t affect my keyboards or typing suggestions, luckily)
Voices can be configured within the Accessibility settings and spoken content, they are defined per language.
My German one was set to Siri, so that was a no no.
Another no no was me changing the voice in MRA.
The same voices are available in this spoken content menu.
If I select a voice here, or change it here AND uninstall and reinstall MRA to get the default voice setting <unknown> there, the spoken content is already way louder.
I tried Anna and Helen for German, Anna seems a bit clearer for me.
Also not just click on a voice, but make sure it gets a checkmark to be properly activated.
Just did a test ride and now the voice directions are fine.
When using Alpin motorcycle earplugs to filter wind noise, it is on the quieter side, but perfectly fine to use. (Tested at 120kmh, because Switzerland)
To be perfect, the said option to increase spoken volumes a bit as in other navigation would be required.
I was able to fine tune the loudness in comparison to music with the tip mentioned by Marc, but learned to leave the limit at 95 or 100db or otherwise the voice gets limited a bit as well.
Thanks for your help!
Long story short:
There was a config issue with my phone, but a setting to increase volume of spoken directions a bit would be very appreciated.