1 Lumen is considered as bright as 1 birthday candle, so 5000 lumen is 5000 birthday candles. And if we were talking about 5000 lumen in watts, 1000 lumen is the same as a 100 watt bulb. So 500 watts is what your looking at, which can light up about 250 sq ft.
Your 5K projector would do best in the conference room type of environment vs. the 8k, which best suits your auditorium/gym size settings.And they both have adjustable lenses, for both short and long throw. Besides the 3000 lumen difference which declares the clarity and visibility of the picture.
In this type of situation it really all depends on the lighting for the clarity of the picture, but to be on the safe side, the higher the lumens the better quality you will get. So more lumens is better when it comes down to projectors period.
If your outdoor event is after sun set, Yes. No projector works in direct sunlight, period, and depending on the time of day the lumen will matter. So, 5k may just be enough but for the best quality of picture, 8k and up will ensure that your picture is clear and visible.
There are a few factors that you have to consider for installing projectors in these environments. First, the angle of the direct light entering the room and the positioning of the installed projector. If the screen is positioned where light from the window hits it, then you will have issues seeing your projected picture.