The main problem with road bikes is that they normally don’t have any prepared place to attach bags. Some people solve this by carrying everything in a small backpack. This could work, but your shoulders may suffer after a week on the road. I have shoulders like an anchovy, so I put my stuff in bags attached to my bike. Fortunately you can always attach a front bag to your handlebars.
I have actually toured the Alps for a week with only such a bag! It is a big challenge to get everything you need into it, but it can be done. Some such bags have a small built-in lock that lets you lock the lid and itself to the bike. It won’t stop a thief equipped with some simple tools, but probably enough security while you’re having lunch. Remember to also bring along an extra key key if the original is lost. Store it somewhere separate from the original.
For most trips you also need another bag, and I have tried two different designs, both attached to the saddle post. My first had a solid metal grip which you buckled onto the saddle post, then the bag itself was attached on top of a rack extending back from the grip. Solid, doesn’t swing, but heavy.
Today I prefer a soft bag which straps onto the saddle post and also attaches under the saddle with clever straps. Much lighter, and still big enough for my needs. The only drawbacks are that these bags swing a bit back and forth while climbing uphill, and that it is somewhat cumbersome to get anything out of it while attached to the bike. So only stuff such a bag with things you don’t need during the day. What is needed during the day depends a lot of the weather, so I make the final decision each morning what goes into which bag.
Explore your options online or at a well equipped bike shop. Pick bags that can take some serious rain and are big enough for your needs, then adjust your packing to your bags. First pack the things you must bring along, like bike repair equipment and toothbrush, then all optional stuff. The bulkiest things are normally that warm sweater, rain jacket, spare clothes, tires, tubes and extra shoes. You absolutely won’t be dressed like James Bond at that evening restaurant, but a pair of summer shorts and a shirt/blouse can get you a long way, except they’ll be wrinkled from the pressure in your bag. Us bike adventurers are perhaps the least well dressed bunch of people.