A warm welcome to Mac-Forums. Thank you for your post.
Others may have additional thoughts and suggestions, but here are mine.
128GB is a very small amount of storage, but everyone understands that money doesn't grow on trees.
So accepting that situation:
1. Is it essential to have Windows 10 on a separate partition? I mean how essential is Windows to you as opposed to just certain apps like Word, PowerPoint, Excel etc? Because you could get all these and more in different ways:
There are free apps which offer Word, PPT and XLS-like features. There is a 365 subscription service to Office for Mac (costs a monthly fee, or annually for a bit less - maybe beyond your pocket?) which would take up less storage space AND give you 1TB of free storage Cloud space -
https://products.office.com/en-GB/compare-all-microsoft-office-products?tab=1
2. You could move your iTunes and Photos Libraries to an External Hard Drive (EHD) and run them from your Mac - might free up a lot of space if you have a lot of Music or Photographs/Videos.
3. You cannot extend your Mac's Internal Hard Drive (IHD) in the way you mentioned. Better to keep big storage items on an EHD, as I suggested and run them from your Mac. (and NB my comment about Cloud storage, if you're comfortable about that).
4. What other apps are taking up the bulk of your storage space other than those things I've mentioned already?
Ian