Pool Pump/filter sizing question, options??

I have a pentair full rated 1 hp single speed whisperflow pump that is dying and need to replace the motor. I have it hooked to a pentair tangelus 60 sand filter and a Pentair IC40 salt chlorine generator,Never can get pressure in filter below 19 psi, even just replaced the gauge, usually backwash at 25 psi. May be geting some sand into pool.
Pool is a 28,000 gallon vinyl liner inground. Pump runs 24 hours a day.
Was thinking maybe filter is to small or pump to big. Though about replacing motor with a 2 speed 1 horse full rated with a toggle switch for High/low and a mechanical timer.
I would then just use the high speed for backwash/ sweeping. How many hours a day would i have to run the thing on low speed?
If it matters i also have a extendaswim wood heater I used in the spring and probably next weekend that i can bypass.
Motor is shot, bearings bad and housing rusted through.
I have only owned the pool since december, came with the house.

Thanks

Looks like you'd be better off replacing the motor, seal, impeller and diffuser . . . and downsizing to the Pentair 3/4 HP 2 speed motor. The 1 HP model is significantly oversized for that filter. At 19+ psi (48 ft) + maybe 20 more feet for piping upstream of the filter, the pump is delivering maybe 80 GPM.

But, maximum flow rate for a 24" filter (15 gpm per sft) is 60 GPM (Pentair specs), but you'll get better filtration at 50 GPM.

If you downsize, you'll still be running too high on 'high', but hopefully, you'll run on low unless cleaning.