best option in my opinion is serialization and hexadecimal conversion.
for example, a player has an inventory with 100 gold pickaxes and 200 diamond ones.
you can represent this as simple as *1:100*2:200. * is the identifier between items, the following number up to the colon is the ID, and the number after the colon is the amount.
you can even go further and turn your IDs and quantities into hexadecimal which would look like *1:64*2:C8.
(id of 1 is for gold pickaxe, 2 is for diamond)
there is another step for people who need even more compression, but i’ve never found it necessary. roblox gives you >4 million characters to store data in, and with my previous example you just stored 300 generic items in only 10. plus you have the added bonus of being able to read the serialized data and understand it without having to run it through a decryptor.
if anyone has other steps id love to hear them.