Don't overthink this.
Just create a new account with the name you want.
Then, you can "manually copy" items from the old home folder into the new one, WITH THIS CAUTION:
You CANNOT copy the folders "immediately inside" your home folder (such as "movies", "music", "pictures", etc.
However, you CAN copy whatever is INSIDE those folders.
For example, if there are pics in your "pictures" folder, do this:
1. Open the OLD "pictures" folder
2. Select one, or more, or all of the items inside it.
3. Drag and drop them into your NEW "pictures" folder in the NEW account.
Give this a try.
It's easy and you will "lose nothing", because those items you are copying will remain in the old account until you delete it.
Create the new account, copy things as required, and run with the new account for a few days. If all looks good, then you can delete the old account if you wish.