This mod adds a set of commands which open a simple NBT editing GUI; literally a text editor with the raw NBT of the thing you are editing

WARNING; SAVING EDITED NBT TO AN OBJECT CAN CAUSE CRASHES AND WORLD CORRUPTION IF YOU MAKE A MISTAKE!!!
If you're not sure, make a world backup first, and be extremely careful about who you let use this on a server (command is normally only accessible for OPs)

/nbtman hand
Edits the item in your main hand

/nbtman self
Edits YOU

/nbtman nearesttileentity
Edits the nearest tile entity

/nbtman nearestentity
Edits the nearest entity (not you)

IF YOU SAVE NEW NBT TO AN ENTITY, AND THE NEW NBT CONTAINS A POSITION, IT WILL SPAWN THERE INSTEAD OF WHERE IT IS NOW. YOU CAN SAVE A TEMPLATE AFTER DELETING THE "Pos" BLOCK IN THE NBT TO PREVENT THIS WHEN LOADING THE TEMPLATE AND SAVING TO AN ENTITY

Once the GUI is open, you can edit the raw NBT and save it to the object.
You can also save and load NBT templates, on either the server-side (shared) or the client-side
Note that when playing single player or when hosting a LAN game from client, your "server templates" and "client templates" are the same thing, so you'll see all of them in both lists

There are a couple of basic examples in the images section. You can do some much more interesting stuff with it though; I suggest using the command to open up items / entities / tile entities and see what data they have stored in them. It will give you a better idea of what you can mess with.

Yes, you can use it in your modpack if you want (MIT license).

A list of my curseforge mods can be found here

NBT Manipulator