Alternatively, you can simply uncap theirs. It's a fair game and it isn't one-sided

Good idea but the implementation process would be highly risky to store an inventory database in our system whenever a player is offline as crashes, rollbacks will all cause the inventory saves differ from what should be there in the offline acc's inv in the first place so unfortunately I don't think this is a feasible idea to implement.

Higher minecraft versions has much more issues than the stable versions prior to 1.13. Prisons and basically all other gamemodes we ever do will almost most likely be done on 1.12.2 and no other versions higher (Unless ofc we do like a survival server) -- Anything version related is easily patchable with ViaVersions for the most part, as such this really isn't something we will be looking into at all.