If you want something lighter weight then photoshop currently us, you can use photoshop cs 2, that should be free/freely available
you could also use clip studio paint, the thing has a manga work flow in it already, also its more or less photoshop sans the in depth photo manipulation aspect.
you also have affinity photo, its more or less a competent photoshop replacement. can't tell you it would be lighter weight, but it has no cpu requirements listed, just 2gb required 4gb recommended.