cassette nerd here.
cassette for the flat: 12-25 11s, or equivalent. the shimano one used to be 12-25, not 11-25, from memory. but it's awesome, i abused it so much on the flat. you always get the perfect cadence, it's wonderful.
Alternatively, do 11-28, if you have the occasional hill. it's still good, but not as good.
definitely dont get more, 11-32, 34, 36, if you dont ride your bike uphill makes no sense, because YES, it IS better to have a tighter range of gears, it does matter.
But obviously, if you get 12-25, dont show up to the mountains. it's very easy to swap a cassette though, you just need the 2 tools, or get a friend to do it, it takes me under 5min for sure.
And yes, you can get it from aliexpress. for the flat, weight doesnt matter, so get a full steel one, standard. shimano is the standard. it should cost you ~25$ shipped.
dont get aluminium, not relevant for your use case.
no need to pay extra for a super light one because you ride on the flat.
https://www.aliexpress.com/item/1005006695084037.html11s 11-25 there, i see at 20 eur w shipping to europe. i'd get that one, or equivalent.