I think the best time is also tied to the reason for trip? If there is one. baseball spring training halloween, xmas, springbreak flower/garden festival/ food and wine in fall. These events usually last several months, so you can then also look at school schedules and cross reference how busy parks will be.
Our trips - Late may, first week june. Nice weather, longer days, kids may or may not be in school after memorial day. good fishing if not to hot already. Our last trip in mid june, we determined that another summer trip was out, simply because of people congestion. I dont mind a good crowd, but when I have a choice, I will park when school is in and crowd level is lower. So late may 15-30th works greats and hope for early summer heat wave. Additionally when you go in the value season your campsite rate is lower!! meaning more days at FW.
Late Oct / Dec - Weather can change drastically with strong cold fronts. Dark earlier, floral/plants not all popping like spring. Awesome holiday decorations, Fort is decked out by campers. Pool time can be chilly, water is usally heated.
When time frame matters - Take xmas - first 2 wks dec great time, low park traffic. 3rd week moderately busy, then 4th wk dec 24-jan 1, parks are at max capacity. we dont even think about even going to parks.