Murcielago Is there some kind of guide with key points on what exactly is helpful to test?
No, not really. Sometimes there'll be specific things to check, but I am not aware of any this time around. Maybe calling and/or texting stuff if you care to put a SIM card in your test device. It's a big release, so there may be strange issues with random things. So, basically, I'd say test as much as you can or want.
Murcielago Since I am unsure whether log files contain sensitive data, I would prefer to email them (or send them in private conversation) instead of posting them on the Matrix channel (the question is of a general nature and relates more to my daily driver device, not the spare one) - is that still helpful or more of an obstacle?
If you encounter a problem, I'd say the easiest thing to do would be to post about it here or in the testing room on Discord/Matrix/Telegram. If someone wants logs, they'll ask for them and you can either send them to someone privately, or someone will ask for logs to be sent to the generic email address we have for logs.