What to pack for small 500km touring?
You need to be very careful with the weight limit on that rack, it will flop about if overloaded a little, or break if overloaded too much. But you're riding short distances in heavily populated areas, so presumably you're staying in buildings and buying prepared food, so you don't need to carry much. If you can afford to spend a little extra money on buses etc, there's very little risk so the best bet is to try it and see.

As Renesis pointed out in comments, while google maps is pretty good at navigation, you do need to select the "bicycle" option rather than the default car one. Otherwise you'll be stuck at the first autobahn you come to when you get to the "no bicycles past this point" sign :) But your route is likely to be somewhat longer than 500km, but significantly more scenic.

I would take a windbreaker, long pants and a shirt, underwear, toiletries, phone charger, phone and wallet. That's about it. Wash your cycling knicks every night and dry them on the end of your bed if there's no other facilities. If you have light cotton pants and shirt they crumple up to very little space.

The main thing with your first tour is that you get out and ride. If something goes wrong jump on a bus and either continue your tour as a bus-traveller, or go home. It's no big deal, the idea is to learn what works. The big planning exercise is probably going to be booking accommodation :)

I suggest reading random cycle touring sites and travelogues to get an idea of what people do and how they travel, in the area where you're likely to be riding. You have everything from the Paris-Best-Paris style randonneur "carry nothing" tourists to four wheel "bicycle campervan"travellers in the low countries of Europe, so there's a lot of different options you can choose. But the easy one is definitely how you're starting - get a bike, pack a change of clothes, and ride.