1.15 Server Client synchronization

Hello!

I have been working on a server only mod for the past while but now with the team who is working on the server we are shifting into a mod that also has a client side.
But I havent used client side things before, and the small description on the docs outlines the things pretty well but I have a few questions.

If I have a custom GUI for example do I need to look into packets to sync things up?
I want to replace the normal inventory with an inventory that doesnt have the armor and crafting slots, but has different slots instead for example, or furnace like things where you put in some items and a progress bar moves and something happens, but I want it to be instanced by player (so that other players cant interact with your thing).

Or if there is custom information about the player and I want to display it client side in a menu. Like the cold resistance of the player (custom combat/rpg system is mostly in place on the server), do I need a custom packet handler for this, or is there some built in mechanism to facilitate syncing values?

Also, very curious about anti cheat methods, how can I ensure that all items remain un-changed and only come from my item factory, and the client has no way of changing them?
I already serialize the items with a UUID and store their state on the server, but that uuid is stored in item NBT not sure how safe is that, but also any other anti cheat catches I should be aware of?

Thank you!