I've just come back from a tour of Netherlands/Germany including stops through Cologne.
The cycle track network in Germany isn't as good as the Dutch, but it is streets ahead of ours and you'll be on segregated, good quality cycle routes nearly all the time. I wouldn't bother wearing a helmet if I were you (I did have one with me but just ended up using it to store sausage rolls more than anything else. Just beware that you'll have to stop and cross the road a few times.
Also worth bearing in mind is that Germans permit turning right at junctions even when the green man is showing on pelican crossings, so it is worth checking that the lorry or BMW that's coming up behind you is actually going to stop before you cross the road. I never had any bad incidents but worth bearing in mind.
I fitted aero bars to my bike for this tour (review:
https://yacf.co.uk/forum/index.php?topic=108887.msg2308138)), and they were brilliant in both the Netherlands and Germany. Highly highly recommended! Just make sure you get comfortable on them in the weeks ahead of the ride. Because as mentioned, the flats are really, really flat.
Keep an eye out for German cycle shops, many of them have a pump and inner tube vending machine out the front for cycle tourists. In this respect they are actually ahead of the Dutch, I didn't see a free use bike pump when I was out there at all.