that allowed Gate -> Source current was 62 A.
That's allowable Drain -> Source current, with the stipulation that the Gate/Source voltage is 10 V. This tells me that a 5V driver will not "open" the MOSFET fully, and you will see a higher Rdson than the specified value. That being said, if your solenoids are just drawing a few amps, that probably doesn't matter much.
But isn't lower resistor -> more current -> better switching? Please, correct my logic. Or did you mean low current?
You need both high current AND high voltage, and the high voltage is more important than the high current for large loads, whereas the high current is needed for quickly switching on/off.
If drivers cant work 100% on cycles and optocouplers are too slow for PWM, is there any way to have both, PWM and 100% on cycles?
There are drivers that can work 100%, as long as they are only used as low-side drivers. Looking at your schematic again, I think that's what you're doing, so I think you're good, as long as the 7.5V can be used as your gate voltage.
The kind of driver I'm talking about can generate a higher gate voltage (even higher than your VDD!) by charging a capacitor, but that capacitor slowly discharges and thus it needs the down-cycle to re-charge.