The only thing you can do with your PC is to make iTunes backups instead of iCloud backups, and backing up a 256 GB iPhone might require more HD space on the PC than you have available (life was so much simpler in this regard when everyone had 16 GB phones).
There are two major factors that might affect your backup problem - the size of the backup and/or the quality of your wifi connection.
If the issue is the size of the backup (which I suspect - a 256 GB phone can hold an awful lot of stuff)
Go into iCloud Settings > Manage Storage > Backups > This iPhone. What is the Next Backup Size?
On that same page, what are the sizes shown for Photos and Messages (if any - if you're using iCloud Messages and/or iCloud Photos, those categories won't appear there)?
If those sizes are large (let's say 100 GB or larger Next Backup and tens of GBs each for Photos and/or Messages), then you have a very large overnight upload requirement. You'll need optimal wifi performance wherever you charge your phone in order to backup in a reasonable amount of time. It could turn out that there's a source of local radio/electronic interference near where you charge, and/or the signal is weak - either of those would slow you down. Try charging/backing up in close proximity to the wifi router to see if that improves things.
You may or may not want to use iCloud Photos or iCloud Messages for other reasons, but one thing those features do accomplish is reduce the size of the nightly backups, sometimes dramatically. Smaller-sized nightly backups = smaller/faster overnight backups.
Why does using those features help with overnight backups? When you use them, your new photos/messages activity is constantly synced/updated with the photos/messages already in iCloud. Photos/messages are excluded from the nightly iCloud backup because they're already in iCloud.
Example:
I have an iPhone X and an iPad Pro. I'm using iCloud Photos and iCloud Messages on both.
iPhone X Backup Size is 1.76 GB, iPad Pro Backup is 2.5 GB.
Photos (in iCloud Photos) 101 GB
Messages (in iCloud Messages) 5.7 GB
Note that there's only one entry for Photos and Messages, because the iPhone, iPad, and my Macs share those iCloud-based Photos and Messages archives.
