I can pack for nearly any length trip in what is now the extra carryon size bag, but I have a 3-week trip coming up this summer and to add baggage would be the same price as my flights (around £100), so I'm determined to make only a 40x25x20cm work.

I'm willing to do washing along the way, but I'm paranoid about being asked to fit my bag in the airline bin even if I've measured it full because they honestly look so tiny. So I don't want a bulging bag.

I also don't mind buying things along the way, but I still need to carry these extra items from place to place and I have 2 flights halfway through my trip.

I can pack as few outfits as possible, but how/where can I pare down the following items:

Pyjamas - I'll be staying in some dorms so would rather not sleep in underwear (I'm female) nor use up clothes I'll be wearing out.

Shoes - I plan to wear canvas shoes on travel days and in case of cooler/drizzly days/nights, plus take sandals and aqua shoes (the latter doing double duty for beach days and around hostels/in showers). Maybe I can leave the canvas shoes and hope it's always warm? But I've had sandals break while travelling... (Had to wear my aqua shoes all around Pompeii once as sandals were too slippery for Naples streets, other sandals had broken, and canvas shoes had run red dye everywhere in a downpour lol).

Towel - I can buy a towel but it can be hard to find where to buy one right away, plus I still have to carry it around.

Toiletries - absolutely happy to buy sunscreen, shower wash, toothpaste, even foundation once there, but still have to carry it around. I need to travel with saline solution for my contact lenses (100mL bottle) so can't limit my liquids bag to completely flat.

Extra bag - small foldable cloth bag for carrying said towel and toiletries around plus food for journeys. Worst case, I can book it as extra baggage just for the final flight home if I want to keep any extras I've bought along the way.

Handbag - cloth sling handbag that I use day-to-day but will have to pack in my main bag for flights. This bag fits my normal daily things plus water bottle, powerbank, and extra drinks or food that I pick up while out.

Jacket - Thin fabric jacket as I have some late arrivals and very early-morning flights, plus I can hide a small bag under my armpit for flights maybe, but this needs to be carried around the rest of the time.

Chargers - I usually travel with a UK USB plug (I'm a few hours from UK airports), EU USB plug and powerbank. These are all quite bulky so can make my one bag stick out in one dimension in outside pockets or take up valuable room in main compartment.

Pockets - I'm a petite female and none of my clothes have pockets worth storing anything in (at least not securely). Would it be worth investing in baggy cargo pants for flights? Another thing to carry around though when it's too hot to wear them, plus will airlines complain if it's obvious my pockets are loaded?

(Unfortunately I'm quite temperature-sensitive, so it's hard for me to leave out a jacket and ideally pants for when it's cooler also to have to wear said jacket and pants when it's hot...)

Please give me whatever advice you have. Or am I just not a good candidate for onebagging and should fork out for extra carryon?

Bonus advice request: What do you advise for the most space-saving clothing? I normally do summer trips with 2-3 pairs of denim shorts (plus a couple of light skirts/dresses). Can you recommend anything that's smaller to pack and will similarly go with any tops?