Too many items in the end is causing my client to disconnect from my realm

If you're willing to potentially lose all of the dropped items in the overworld as well, you can get two repeating command blocks and force them to run the following commands repeatedly:

/scoreboard players tag @a add inEnd {Dimension:1}
/execute @a[tag=inEnd,c=1] ~ ~ ~ /kill @e[type=item]

This adds a tag to any player in the End, and executes a command to kill all items on them. Following this, your plan of action should be:

1. Grant another account temporary admin
2. Use that admin account to set up the two command blocks
3. Log in with owner account (tag is applied, items are deleted. you may need to log in multiple times)
4. Assuming items are deleted, have admin account break the two command blocks
5. Run /scoreboard players tag @a[tag=inEnd] remove inEnd on owner account

The ultimate answer here, is what I proposed above. Note, this approach is quite general. If you have a world that is misbehaving on a Realm, for any reason

• Tremendous lag from an out of control farm (too many loose items, or too many entities)
• Tremendous lag from an out of control lag machine
• Commands running amock, causing lag, or getting the game into some other bad state.

Then, the best solution is to

1. Download the world to your local PC.
2. Modify world using any tools at your disposal (in particular, disconnections due to excessive lag are not an issue here)
3. Once the world is fixed up, reload it to your Realm.

For what it's worth, I tried Corsaka's suggestion, but the client would disconnect from the server, and reset the server to a time just at the most recent login.

I also tried repeatably logging onto the server to advance the game clock, to attempt to have all the items decay. In this case, the client would consistently time out, and the game would not advance the game clock.