There is a somewhat hidden search syntax you can use to make Pokémon storage cleanup easier. Below is the search string I use. It helps quickly isolate the majority of undesirable Pokémon, and then you can manually inspect the rest:
!3* & !4* & !shiny & 1-4attack , 1-4hp , 1-4defense & 2-4attack , 0-1hp , 0-2defense & !legendary & !mythical & !ultra beasts & !shadow & !# & !unown & !dynamax
Under the search bar in the Recent section, you can long press on the search circle to save it with a more memorable name (e.g., Cleanse).
! is a negation. 3* and 4* capture each star rating, respectively. Most of the others are special categories. The random ranges on attack, defense, and HP help isolate good PvP Pokémon.
Generally speaking, good PvP Pokémon are ones with low attack, high defense, and high HP. The rationale is that attack influences a Pokémon’s CP value a lot, so low attack helps optimize the CP for battle leagues with CP ceilings.