I just stuff what I need in the jersey pockets. Most of the time it is food, phone/wallet, car/house key, sunscreen, and sealant with CO2. On really long rides I stuff an extra tubie in the pocket just in case. If I ride clinchers I bring a spare tube instead of the sealant.
When it's cold at the start and then it gets warmer, I roll up jacket and gloves and tuck them in one of the waterbottle cages, or if both cages are occupied I stuff those in the back pockets too.

A few years back I tried a seat bag for a while, but it did snag a couple of bibs, and I never really saw the benefit of using it, so I decided to take it out. Never missed the thing.

Mimi I loved your picture of the bike with the rack!