We did a trip to Italy a few year ago and this worked well for us…
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No checked luggage. Just the max size luggage for overhead, plus the max size additional carry-on. We have nice Osprey backpacks that are max overhead size, plus small duffel bags for under the seat. Easy to carry between airport and taxi, or train station to hotel, or whatever. So much better than roller luggage which is a total PITA in Europe unless you have a car to get everywhere. Most people overpack anyways - you really don’t need more than ~3 days clothing, if that, just wash stuff in the sink.
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Minimal cycling kit - plan to wash it every day instead. We took one set each. If this is a tour, they probably offer daily wash at the hotel. Or, wash yourself and use the towel suggestion above.
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We did NOT bring our own saddles - our daily mileage was ~50, so figured we’d just make do with whatever was on the rental bikes.
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Don’t forget you CANNOT bring CO2 in carry-on, so either buy a few cartridges at destination or pack a mini pump. Check with rental company to see if a repair kit/saddle bag is included. If you’re doing supported rides, you need way less emergency repair kit.