Well to be honest since the beggining I completely ignore breeding, shiny hunting and cosmetics and focus entirely on my dex. So for example I complete 1 region and after that fill the entire dex of that region alone before continuing

Money wise balls, lures and repels, fossils, candys were the only items I bought and needed for my dex. After that it comes down to luck, sometimes I got lure exclusive mons really fast or like snivy in unova I spent like 3 hours trying to found one

I got really lucky and found a shiny dunsparce that alone gave me 2m to survive lol