It is definitely worth hunting down a disc of Colosseum I still have my disc of the game and also XD: Gale of Darkness both games are excellent and I replay both games twice every year with different teams.
There are two ribbons available in both games that you can obtain for your Pokemon the National Ribbon (This is awarded when your Pokemon is purified) and the Earth Ribbon (This is awarded for winning all 100 battles on Mount Battle with the same 6 Pokemon and not switching Pokemon at the PC) both of these Ribbons award titles to your Pokemon in Sword, Shield, Scarlet & Violet.
National Ribbon - The Triumphant
Earth Ribbon - The 100 Times Victorious
You can get Ho-Oh on Colosseum by defeating all 100 trainers on Mount Battle in Battle Mode (You need to have purified all 48 Shadow Pokemon)
XD: Gale of Darkness allows you to obtain the Johto Starters Chikorita, Cyndaquil & Totodile with their special moves (Frenzy Plant, Blast Burn & Hydro Cannon) you are given the option of which starter you want when you defeat all 100 trainers on Mount Battle without changing your team basically the same requirement for the Earth Ribbon) you are able to get all 3 starters but you will need to defeat Mount Battle 3 times to get all 3 of them.
All the Pokemon you snag and purify in Colosseum & XD: Gale of Darkness can be sent to Ruby, Sapphire, Emerald, FireRed & LeafGreen which as far as I am concerned makes Colosseum & XD: Gale of Darkness main series you can even send the Pokemon all the way to Scarlet & Violet.