As far as i'm aware the effect time of festival food is fixed based on the rank of festival food itself and is independent of cooking and you can buy/sell the picnics you make as well. I'm not sure if the time is based on the person who create the picnic though or the person who places it.
Also even though I am 10k total, I'm not one shoting things in Peacca Abyss and neither is Kenero who is about 3k total higher than me. So food buffs still have a place regardless of what some elitists may say.
Considering the stat cap is 1500 and I can get almost 1300 dex through leveling and my gear, the maximum of 200 increase in dex as well as the fact that trans can easily close that gap makes festival food a bit underwhelming for people with soft capped stats. I'd be alot more interested if it instead gave flat max. However this doesn't mean its not something I wouldn't use, I just dont feel its super amazing.
On another note, they need to increase the stat hard cap soon because its becoming commonplace to reach the hard cap and I feel I'm wasting stats and gear whenever I hit it....