Dress for a cool morning and a warm afternoon
Choose the midday outfit first. Then solve the cooler start with a layer you can take off and carry. This works well on a mild day because the pieces still look intentional after you leave the jacket over a chair or in a bag.
Build a base you can wear on its own
Try a dress or a tee with straight trousers, then add a jacket or cardigan. Check the neckline and sleeves without the layer on; if you would not wear the base by itself at lunch, it is the wrong base for this plan. A light layer in a related color makes the change feel natural. Start with familiar shoes that work for both parts of the day.
Think about where the jacket goes
If you will walk all afternoon with a small bag, a foldable cardigan may be more useful than a stiff blazer. If you can leave a layer at your desk, you have more room for a heavier jacket. For an early wait at a stop, choose a layer that closes. Avoid adding two pieces that both need carrying later.
Run a two-minute test
Leave the base on and carry the removed layer while holding your bag. Try stairs or a doorway. If you need both hands just to manage the outfit, change the carry plan now. Pack a light layer again for the return trip if you will come home after dark.