Will iPhone data be lost when joining Family Sharing?

So if I understand this, you will be making your own account. One of you will set up Family Sharing and be the Organizer. It can be either of you.

When you sign out of the old account you will be asked if you wish to delete the content currently on the iPhone. Say no. You want to keep your content. Read what this does --> What happens after you choose Sign Out But Don't Erase - Apple Support

If you were using Photo Optimization, turn this off so full quality photos are kept on the iPhone --> Manage your photo and video storage - Apple Support

Also make sure you are signed out of Find My with the old account --> Turn off Find My on your devices and items - Apple Support

Read this document to learn how to sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your devices - Apple Support

For an Apple mobile device:

1. Go to Settings, then tap your name. 

2. Scroll down to the bottom.

3. Tap Sign Out.

4. Enter your Apple ID password and tap Turn Off. (Read this if you If you forgot your Apple Account password - Apple Support ) 

5. Turn on the data that you want to keep a copy of on your device.

6. Tap Sign Out.

7. Tap Sign Out again to confirm that you want to sign out of iCloud.

You can then create a new Apple Account for yourself --> How to create a new Apple Account - Apple Support

To learn how to set up Family Sharing, read this support article --> How to set up Family Sharing on your iPhone, iPad, or Mac - Apple Support

Note that despite doing all the above you may still lose some content if it was specifically associated with that old account. Some data are Apple Account specific and will not transfer.

If you wish for somebody at an Apple Store to help you, set up a Genius Bar appointment. Go to this page to find your nearest Apple Authorized Service Provider (AASP) or Apple Authorized Distributor (AAD) --> Find Locations

How to make a Genius Bar appointment

1. Go to Genius Bar Reservation and Apple Support Options - Apple

2. Go to "Get hardware help".

3. Select your hardware and continue selecting options requiring replacing parts until you see "Bring in for Repair"

4. Sign in with your Apple ID

or:

Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone ➞ Contact Apple Support - Apple Support

No. Back it up anyway.

(262474)

Note that officially Apple does not support account merging or splitting. I cannot predict what will happen at the store but they may just tell you to wipe the device entirely and sign in with the new account. That is the by-the-book solution --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support It is unlikely you would have received the solution I posted earlier if you had asked at an Apple Store. ;-)

The problem with that is data loss. However, when you try to do the workaround solution you can end up with issues because the system is not designed for people to do workarounds. Your issue reminds me of this one:

https://discussions.apple.com/thread/254989147

"Can't change Apple ID account photo, keeps reverting to the old one again

I experienced this issue with my MacBook Pro. After wrestling with this for several days, I realized that the photo I was seeing was the one for my Mac User account, not the Apple ID account. When I updated the photo for the Mac User, the problem was resolved."

From a post by sberman at https://discussions.apple.com/thread/256198933?answerId=261712304022

“Apple uses two distinct photos:

Your Apple Account photo. This is set at Settings > [your Apple Account] > Personal Information. You may change the photo at the top there.

Your Contacts photo. This is changed in the Contacts app at My Card. You may edit the card and change the photo there.”

You can still do family sharing. The problems you are encountering is because although account merging/splitting is 90% effective, you're also running into the 10% where is is not or is confusing the system.

Notes may be associated with a specific iCloud account, but I believe they can also be part of email accounts. Frankly I do almost nothing with Notes so I am note sure of the details.

According to these, calendar sharing should be possible:

Share calendar accounts on Mac - Apple Support

https://support.apple.com/guide/calendar/share-calendar-accounts-icl27527/mac

Add or delete calendar accounts on Mac - Apple Support

https://support.apple.com/guide/calendar/add-or-delete-calendar-accounts-icl4308d6701/mac

I personally would avoid signing in and out of accounts. Apple has not designed the system to accommodate that and anything you wish to do that involves items on another account should be done from a device or computer account signed into that account. Share items with Family Sharing to the extent that a service is set up to do that. That particularly goes for iPhones. With computers you can set up multiple users and have a different computer user account using a different Apple Account. Unless it is something designed to be shared and is accessible by other Apple Accounts, go to the other computer user account to do what you need to do.

Sorry, that is way beyond me. I don't use Calendars at all.