We'll be continuing to add more Trainers and other artwork from now until, basically, we somehow run out of everything we can use. For the moment we're effectively working backwards by focusing on the new Masters and Pokémon Scarlet & Violet content - and anything we're still missing from Masters we'll eventually pick up as we continue working through it.
Official artwork of characters in Pokémon isn't really as complete, consistent or in as high quality as the Pokémon artwork is (for example, although we've been adding every new piece of character artwork for Scarlet & Violet as it is released, I wouldn't expect us to have good artwork for even all 8 gym leaders for some time after launch). So there's enough extra work to collect and curate these for future updates that they end up naturally becoming lower priority than the more available and usually better quality assets from Masters. But we'll get there eventually.