Worth upgrading to longboard deck?

Hi! I built my board with a cheap skateboard as a base, upgrading the wheels to Orangatang Kegels. This is my current skateboard: Link to Decathlon

Would I get noticeably better ride stability if I bought a longboard deck? The deck is already 31.5" long

Stability is mostly by the trucks and your bushings. The deck is basically how you would balance on the board during high speed. If you can’t get your front foot as close to the front trucks, you’re going to need a deck for that. Usually stiff decks are the best to get. Longboard decks are just usual standards for high-speed since it’s stiff.