Do I use Radiant Venusaur's "Sunny Bloom" Ability before or after taking prize cards at the end of my turn?
Sunny Bloom is used at the end of your turn after attacking and checking for a KO but before Pokemon Checkup. So if your attack causes a KO, you would take Prize cards first, and then get to use Sunny Bloom.
Source: TPCi Rules Team (2022-08-04)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH105_4
SWSH105_4