Don't go any further until you have the screen size issue resolved.
That, or scale up your playable character.
Hard to imagine a platformer gameplay setup where having such small protagonist compared to such a large scene will work well. Kinda feels disconnected being so "far away" from him.
Michelote is probably right - tiles are the way to go here. If you try and go HD pixel art, you might go insane.
uhhhh you know what, hold on, lemme get an edit . . .
*edit do*
Suddenly, an edit:
Changed to 2x style. This cuts the pixel creation workload to ONE FOURTH (and looks cool).
Changed screen aspect ration to 16:9 so there's no black bars on most monitors when full screen.
Tweaked muddy palette.
Added more bg parallaxing layers.
Added bg fog for some environmentally feels.
Darkened ground tiles and closest forest canopy to improve visual organization. Put more focus on PLAYABLE area.
Added The Red Dreadnought Knight of the Order of the Sacred Brothers, protector of random treasure chest.Compare before & after: