Strings as lists

You should be able to use append, prepend, and insert on Strings in the /data command (and also other commands).

Examples:

• /data modify entity @e[type=armor_stand, limit=1] CustomName insert 1 value 'H'
• /data modify entity @e[type=villager, limit=1, sort=random] CustomName append value 'F'
• /data remove entity @e[sort=nearest, limit=1] CustomName[3]
• /execute store result entity @s CustomName[3] byte 1 run scoreboard players get @s kills