When a relay spec says that it is a 10A rated relay, that is the absolute maximum current allowed and larger currents will immediately damage the relay. As you noticed, the contacts are degraded by switching higher currents, so if you also want long life, you need to substantially overrate the relay switching current.
Another problem that shows up in your datasheet above is the ability to switch inductive loads like motors. Your 10A maximum relay can only safely switch a 3A motor (and this another absolute maximum). If you are switching a motor, the best spec to check is your motor’s LRA or locked rotor amperage. This is the current that is drawn briefly when the motor starts up and you want to handle that full current with your relay. Not being careful here will cause the relay contacts to weld together and you will not be able to shut the motor off with the relay, which can be dangerous.
The best way to approach this is to figure out what load current you are switching and then select a relay that is spec’ed to handle 3-4X that current. Working backwards is more difficult.