Fully Improved: Gustav achievement in Spiritfarer: Farewell Edition

No spoilers. If you got stuck try visiting Ambertown Park's park, or/and Oxbury's cafe (maybe a few times).
Sooner or later you'll get the blueprint to improve his gallery, and eventually, he's going to describe what else must be upgraded.
TLDR: Stick to his quest-line and if it stops making sense visit Ambertown Park.