Swift Sport 1.6 manual, 450+km in mixed condition before low fuel warning lights up.

No, that's a bit too thirsty. At least I expect to reach 360-380km when the warning light comes up. But FC is highly dependent by driving style also.

Are you sticking to the recommended minimum tyre pressure of around 200-220 kpa ?

Too thirsty for such small car with  low powered engine. Should be at least 10km/ltr. My Sport is at 10.5-10.7 on the digital meter but if measured manually per 1 tank sometimes can reach 11-12km/ltr. Constant highway drive should easily reach 430-460km.

Try do carbon clean then top up with some fuel system cleaners (Gumout, 3M, Redline, Caltex,  or Bluechem). Make sure your spark plugs are still in good condition. Try use 5w30 oil viscosity if you have been using thicker oils.

What do you mean with "car leg"?? Suspension, rims, tires??

Which STP? You need to use cleaners that contain PEA otherwise not as effective. Those brands I mentioned above got PEA while Bluechem able to dilute water content. STP not all have PEA, I only saw 1 product from STP that states got PEA content, in a silver/grey bottle.

Change absorbers no impact on FC. If you change to bigger and heavier rims/tires then will impact FC.

mine same model. can get 350km per tank for 100% city drive. long journey can get 450km. sometimes reach 500km if drive speed limit
- iridium spark plug
- engine oil semi 10w40 ( now use eneos, service every 5k km)
- ATF, suzuki ori oil, service every 20k km
- tire use pressure 230kpa( follow manual), standard rim and tire size, got TPMS to monitor