Public domain art Part 1: www.skeletoncodemachine.com/p/public-dom...
Public domain art Part 2: www.skeletoncodemachine.com/p/more-publi...
Public domain settings: www.skeletoncodemachine.com/p/public-dom...
5 free fonts: www.skeletoncodemachine.com/p/five-free-...
@exeunt.press has some good guides on free public domain art, colors for print, and (maybe, i might be misremembering) fonts.
I also just read @danielsolis.bsky.social’s book on graphic design for board games, but its very board-game specific so I wouldn’t rec. it unless your RPGs are board-gamey.