Sweaters in spring/summer?

Light cotton sweaters are fine in summer. Even light cashmere may work but I have not personally tried it.

Sweaters are fine, just dress according to the weather. If you want to coordinate for the hotter weather, lighter fabrics like merino, silk, cotton, linen etc. are preferable, as well as lighter, brighter colors. It's always good to keep a lightweight merino v-neck around for a chillier day, IMO.

I have a polo 100% linen sweater that I wear on the hottest days. Looks good under a blazer also.

Alpaca breathes well in the summer, although it is a bit scratchy.

Alpaca is way to warm in my experience. My go to sweater when it is very cold is my peter scott alpaca/merino full zip. Incredibly hot. In the summer thin merino sweaters are great for evenings and during spring for most days too. Light cashmere sweaters are fine if you are not doing anything actively. When walking around the city in the sun I find even lightweight cashmere is too hot for me. I dont like cotton sweaters that look like they are made from wool. It loses its shape so fast and feels heavy when humid. Me no likey.

I have a pale pink cotton v-neck from RL Polo that works very nicely with many summer outfits, for cooler evenings. I also have a cream long-sleeve polo in lightweight cashmere from Loro Piana that fits much the same purpose.

As long as you stick to lighter fabrics, sweaters can be very versatile additions in summer.