are there any better - air-folled, foam-filled or gel-filler tyres available for the Zelia? The ride is so bad with the foam wheels, my baby is being shaked by everything on the ground...
Unfortunately, there are no wheels available - not even the original ones - to be purchased separately. As a spare part, the EVA foam wheels that are supplied with the pram are probably achievable... but that is not what you want.
To your question, no air-filled or similar, more all-terrain wheels can be clicked on the Zelia frame. As a budget-friendly stroller system, it was made simple and not all-terrain a tall (also because of the rather weak suspension), and there are not that many extra features or accessories available. You will need to live with the pram as-is and, perhaps, swap it after a few months for something lightweight/compact, maybe mid-size, forward-facing, with better wheels/suspension.
Even if you found similar wheels that somehow would be able to be fitted to the chassis, it might be dangerous as there are no wheels that are compatible (and the axles might just unclick at any moment).