Sinnoh's Pokédex in Pokémon Brilliant Diamond and Brilliant Pearl is much more forgiving and achievable than Galar's in Sword and Shield. As in the original game, there are only 150 Pokémon in the Sinnoh Pokédex. Also, you don't have to "catch them all" - you just have to see them all, through battles with coaches or in the wild. For those who have completed their Sinnoh Pokédex and want more of a challenge, Shining Diamond and Shining Pearl offer an additional obstacle: the National Pokédex.
The National Pokédex is part of the Shining Diamond and Shining Pearl post-game. To get it, you must first beat the Elite Four and become the Champion. You must also have a full Sinnoh Pokédex. With these two successes behind her, she returns to where it all began, at Professor Rowan's laboratory in Sandgem Town. You speak to Professor Rowan. Upon learning that you have completed your Pokédex, Roman will request the appearance of his friend, Professor Oak. Professor Oak will then upgrade your Sinnoh Pokédex to a national Pokédex.
The National Pokédex will add 300 new Generation I, II and III Pokémon to the wilds of Sinnoh. It will also unlock Ramanas Park, where you can catch Legendary Pokémon from these previous generations.