You can play with any deck you like, there are no restrictions (other than the usual 60 cards, 15 card sideboard, max 4 of each card other than basic lands).
The main event for Copenhagen is Modern Horizons Limited, so the deck you bring with you does not matter. If you choose to join the main event, it would be good to familiarize yourself with the Modern Horizons set and the limited environment. The main event will have a high entrance fee and there will be a lot of cash on the line, but it will be difficult to advance to day 2.
Other than the main event, there will be side events running alongside the main event, smaller tournaments with smaller prize pools and limited number of slots some of them will be standard, so you can play your decks there, even if you don’t think they are optimal. The side events during the first day will be typically a bit less competitive (since many of the competitive players will prefer the main event). On the 2nd day, the competitive players who did not advance in the main event will be playing side events too. Many side events will fire per day, so if you do poorly in the first one you join, you will typically still be able to join others.
I suggest figuring out what it is you want to do - if you just want to have fun, I suggest joining whatever event you like, don’t worry too much about being competitive (taking into account the entry fees). If you want to be competitive, I suggest trying the main event. One “fun” event I like to look forward to at GPs/MagicFests is “Chaos Sealed”, which is a limited format with packs taken from random sets. IDK if they still do that nowadays, but typically it will be in day 2.