It's been a bit since the launch of Pokemon Sword and Shield, but a lot of people have been hitting the endgame this past weekend and have asked for some tips. The first thing I always tell them? Go get the Catching Charm if you haven't already picked it up.
Head to Circhester (that's the ice village in the top right hand corner of the map, with the blue aesthetic). Then go right and go into the left-most hotel. Inside, go into the elevator (there's only one automatic floor option), then head to the left and go into the last door. Talk to the doctor and boom: you have yourself a Catching Charm.
Basically what it does is enhance all your critical capture chances. You'll know you hit a crit because the throw animation is a bit different, and you'll capture a creature after one ball wiggle. Every time you capture a new 'Mon, your crit chance goes up, but the charm boosts you to the point where you'll start regularly seeing them even with a light 'Dex.
Good luck catching all 400!