How to attach variables to a player that persist after server reset and the player leaves

[WayPastStupid]

I'm making a 1.21 version of the hypixel pit(IP:waypaststupid.play.hosting) and someone recommended using skript instead of command blocks so i started learning yesterday.
I'm trying to add a killstreak system right now that gives a bounty every 10th kill in a row and clears the killstreak and bounty on death.

on join:
    set metadata "KillStreak" of player to 0
    set metadata "KillsToBounty" of player to 0
    set metadata "Bounty" of player to 0
on death:
    victim is a player
    attacker is a player
    set metadata "KillStreak" of victim to 0
    set metadata "KillsToBounty" of victim to 0
    set metadata "Bounty" of victim to 0
    add 1 to "KillStreak" of attacker
    add 1 to "KillsToBounty" of attacker
    is "KillsToBounty" of attacker equal to 10:
        set "KillsToBounty" of attacker to 0
        add 1 to "Bounty" of attacker
        broadcast "%attacker% has just reached a killstreak of %"KillStreak" of attacker%"

 

[BaeFell]

You'd need to use variables like:
{killcounts::%uuid of player%}
These will then exist when the player disconnects or the server restarts. Metadata is not permanent.