Two important aspects of family sharing to note are that it is to share purchased content, and services such as iCloud space using an iCloud+ plan. Apple summarises this in the support document What is Family Sharing?:
With Family Sharing, you and up to five other family members can share access to amazing Apple services like iCloud+, Apple Music, Apple TV+, Apple Fitness+, Apple News+, and Apple Arcade. Your group can also share iTunes, Apple Books, and App Store purchases, and a family photo album. You can even help locate each other’s missing devices.
To address your questions:
when we upgrade to the family plan...will any of my photos/notes/files get shared to other accounts/members in my family?
No, except where you want to share. You can create a shared note for a shopping list, for example. You can create a shared family photo album. Apple's simple statement in What is Family Sharing? explains this: Each family member uses their own account, so everyone’s experience stays personalized and private.
if my photos/notes do stay in devices connected to my account, will my photos be shared across devices signed into my account (not a family members)?
Yes, as they do before you join a family group. There is no change to this.
I have an iPad and a phone that I want photos to be shared across...my devices, and don’t share across family members
This is unchanged after joining a family group.
