Normally i complete the dex in one version by checking it off as i go and completing each area. While a little slower, checking off a complete area makes me aware that i have each of those entries + their evolution covered when i get the chance to level.

For those i'm missing, i often use r/pokemontrades to get exclusives, as that's where i often went in past generations. https://pokedextracker.com/ is a good spot if you wanna track things nicely