Small update ;)
After replacing a very large nature asset pack with some really fuzzy-detail meshes, and enabling compression when packaging, rebuilding the lightmap for my map and enabling this compression option, Unreal's editor packaged it in under 1GB! So, hopefully I'll be ok... Still a rather large submission, though. I don't understand how itch.io's play via browser thing works exactly but I hope players will have no issues trying out the game.