The general rule of thumb, especially at MCO, is arriving at MCO at east 2 hours prior to your flight (3 hours for international flights).. From WDW to MCO, I would allocate about 30 minutes of travel time, so I would hence recommend scheduling your pick-up at least 2.5 hours prior to your flight departure time..
In regards to availability of Ubers/Lyfts, it really depends on the time of day and demand, generally speaking there are typically lots of drivers available in the WDW area, but once again this can fluctuate depending on the time of day.. I would recommend downloading BOTH Uber and Lyft apps on your phone so you can check both availability and pricing on both, and select the best option available at that time..
There is of course always the option to book a private car through Mears, this would allow you to pre-schedule a pickup time in advance and pay a flat rate for the trip, you would also have a dedicated driver that can assist you with luggage, etc..
And last but not least, of course you have the shared options as well such as Mears Connect and Sunshine Flyer, these are essentially airport shuttle services (unofficial replacements for DME) that charge per passenger and can make multiple stops along the way..