You will have a problem driving a high current DC SSR at PWM frequencies above a few kHz. I would suggest using a proper half-bridge made from NMOS devices and a high-low half bridge driver. A good choice would be the IR2103, which can be driven by a single PWM signal for both high and low side MOSFETs.
And I think you should use a triangle wave and comparator to create your PWM signal. Here is a circuit using three op-amps and a 9V single supply, which could be derived from your 24V battery. You don't really need isolation, but be very careful to separate the high current circuit from the signal circuit. I'd suggest starting with a small 24V DC hobby motor to develop your design before graduating to the 3 HP end project.
I have added a high-low half-bridge driver, an NMOS half-bridge, and an inductive motor load, with a 24 VDC supply. The actual hardware design should add gate resistors to the MOSFETs, and various bypass capacitors on the power supplies, but this shows, in principle, how a DC brushed motor PWM drive should work. The PWM frequency is about 13 kHz, and can be changed using different values for C1.
It's important to note that I added an inverter for the PWM signal for this particular half-bridge driver. Also note that the high side MOSFET drives the motor ON, and the low side turns on to conduct the stored inductive current. It may be wise to add a commutating diode across the motor, and perhaps an R-C snubber, especially for a brushed DC motor to reduce noise.